NCJRS --Probably the most extensive source of information on criminal justice in the world. NCJRS is a group of clearinghouses supporting all bureaus of the U.S. Department of Justice, Office of Justice Programs, the National Institute of Justice (NIJ), the Office of Juvenile Justice and Delinquency Prevention (OJJDP), the Bureau of Justice Statistics (BJS), the Bureau of Justice Assistance (BJA), the Office of Victims of Crime (OVC), and the Office of National Drug Control Policy (ONDCP). For federal corrections, see the BOP.

• Bridging Gaps in Police Data - pdf document by Michael Maltz
• Criminal Justice Focus - Capital Punishment - a good page on an important topic
• Data Analysis in the Social Sciences - a short list of repositories, mostly polling data, from this Univ. of Texas site.
• Government Printing Office - Nearly 10,000 books, pamphlets for sale.
• Institute for Government Studies - IGS at Berkeley is a major policy think tank.
• Interuniversity Consortium for Political and Social Research The ICPSR is a consortium of colleges and universities where researchers have archived their latest research projects. Over 500 online data collections. Good source for secondary data analysis. Recommended.
• Justice Research and Policy (JRSA) Online journal and other information from the Justice Research and Statistics Association. A nice web site.
• SEARCH: National Consortium for Justice Information and Statistics A non-profit organization dedicated to providing cutting-edge information. Most files are viewable from within your browser.
• National Center on Institutions and Alternatives - ranks the states in terms of where it is safest from crime. For city rankings, see Morgan Quinto Press. or visit Money Magazine's Best Places to Live.
• National Criminal Justice Association An association that lobbies for lawmakers to utilize more research findings. Most files are viewable from within your browser.
• National Police Use of Force Database A project by the International Assoc of Chiefs of Police.
• National Victim Center Comprehensive site for victim-related information, such as status of stalking statutes, current lawsuits, and other statistics.
• Research Resources for the Social Sciences - A colorful, easy-to-use guide to the major search engines for social science research, from Carleton University.
• Social Science Gateways on the Net - An extensive set of links to sites providing data on a wide variety of social science topics: poverty-related resources, political reform sites, polling data, and other statistical archives. Huge.
• Sourcebook of Criminal Justice Statistics This is the online version of the annual SOURCEBOOK published by SUNY-Albany. Over 600 tables of data on just about any topic imaginable. Perfect tables, charts, and diagrams for importing into term papers. Recommended.
• State Information Authorities Indexed state-by-state by Cecil Greek. All states have one of these grant and research clearinghouses.
• Statistical Resources on the Web - a useful site from the University of Michigan's Documents Center, links to resources, online databases, and archived datasets of every kind, from agriculture to crime to weather, , with their Comprehensive Sociology section probably the best.
• Syracuse Transactional Records Access Clearinghouse Info on laws enforced and crimes handled by federal law enforcement agencies: FBI, DEA, ATF, IRS, requires password, but free sections include About the Law, a guide to 3,000 fed statutes.
• Uniform Crime Reports The UCR is published by the FBI as our "official" crime rate and is searchable by city and state; this link takes you to the FBI's home page as I got tired of tracking down their data site.

Also, for crime statistics, see Guide to the State SACs or use the Table below:

    The following are various groups, mostly nonprofit organizations, that exert an influence in one way or another on crime and criminal justice policy in the United States.  For the many think tanks which are involved with foreign policy or international affairs, your best site for this is FPRI's Think Tank Directory or any of DrTomOConnor's favorites (immediately below).

Academy of Criminal Justice Sciences (ACJS) - Premiere professional association devoted to scholarly study of criminal justice issues, composed primarily of college professors.
American Bar Association (ABA) - Lobby group composed primarily of lawyers working on solutions to the nation's crime problem.
American Civil Liberties Union (ACLU) - Interest group that uses whatever means necessary to preserve human rights, especially in the face of justice systems.
American Correctional Association (ACA) - Premiere correctional employee association for those interested in professionalism, runs a program which accredits prisons.
American Council for Drug Education (ACDE) - Group dedicated to diminishing drug use and its impact through gathering and disseminating data.
American Judicature Society (AJS) - Nonpartisan group of judges, lawyers, and citizens working toward better judicial selection, ethics, and effectiveness in criminal justice.
American Society of Criminology (ASC) - Premiere professional association devoted to the scholarly study of crime, composed primarily of college professors.
American Society for Industrial Security (ASIS) - Premiere professional association for the private security industry.
Bureau of Justice Assistance (BJA) - Dedicated to system-strengthening, a part of the LEAA-LEEP legacy, and their website contains a copy of the Omnibus Crime Control & Safe Streets Act of 1968.
Bureau of Justice Statistics (BJS) - Operates as a kind of government think tank, employing Ph.D. talent that would otherwise go into teaching.
Bureau of Justice Statistics/Sourcebook of Criminal Justice Statistics - The federal government contracts out the Sourcebook to SUNY-Albany, which contains what are regarded as the most important data tables.
Center for American Progress (CAP) - A liberal think tank devoted to progressive American policies that has close ties to a Democrat-led White House.
Center for Court Innovation - Research on community courts and community-based initiatives from this NYC agency; see the Center's courts page for how family and drug problems are handled.
Center on Juvenile and Criminal Justice (CJCJ) - Nonprofit organization designed to reduce reliance on use of prison as solution to social problems.
Center for Public Integrity - Watchdog group for violations of public trust/ethical misconduct among officials.
Children's Defense Fund - Leading child advocacy group, concerned with every aspect of child safety, health, and crime.
Citizens United for the Rehabilitation of Errants - A popular group interested in all things dealing with rehabilitation of criminals.
Community Policing Consortium - A partnership of various police-related organizations which plays a principal role in proliferating the philosophy and programs of community policing.
Crime Prevention Coalition of America - Nonpartisan group representing a number of different agencies which is devoted to greater freedom, security, and respect for the rights of all.
Criminal Justice Policy Foundation - Conducts surveys and research on solutions to criminal justice problems like drug policy and policing.
Extra-legal Governance Institute - A UK institute devoted to organized crime research.
Federal Bureau of Investigation/Crime Statistics - The nation's oldest form of crime reporting, since 1935, with lots of other reports and publications.
Grassroots.org - An attempt to put together a large collection of 501c3's.  
Heritage Foundation - Nation's major conservative think tank dedicated to preserving America's traditional values.
The Injustice Line - Web page by a private attorney who documents injustices, mostly racial injustices, in the criminal justice system.
Institute for Global Communication (IGC) - Group of radical-oriented social justice organizations with a variety of causes, many of which relate to criminal justice.
Institute for Intergovernmental Research (IIR) - Research organization specializing in law enforcement, juvenile and criminal justice issues.
Institute for Law and Justice (ILJ) - Private, nonprofit research and consulting firm which reviews legislation for impact analysis.
International Association of Chiefs of Police (IACP) - One of the nation's oldest professional associations dedicated to police professionalism.
Inter-University Consortium for Political and Social Research (ICPSR) - An institute at the University of Michigan-Ann Arbor which for years has collected the nation's dissertations as well as datasets of other researchers on the criminal justice system.
Justice For All - An advocate for change group out of Houston aimed at making the criminal justice system better at protecting the lives and property of law-abiding citizens.
Justice Research and Statistics Association (JRSA) - National organization for all the state's Statistical Analysis Centers; each state must have a centralized information clearinghouse to receive federal funds.
Law Enforcement Against Prohibition - Professionals speaking out against the war on drugs.
Milton Eisenhower Foundation - continuation of the Kerner Commission's work.
Mothers Against Drunk Driving (MADD) - A group determined to stop drunk driving and support victims of this crime.
National Association of Attorneys Generals - Promoting cooperation, coordination, and communication among chief legal officers. 
National Center for Rural Law Enforcement - Dr. Lee Colwell's Institute in Arkansas. 
National Center on Institutions and Alternatives (NCIA) -- Nonprofit organization dedicated to improved system delivery; used to publish "Safest Cities in America" reports, among other things.
National Consortium for Justice Information and Statistics (aka SEARCH) - A clearinghouse for criminal justice shareware, online databases, and other means of information sharing and/or standardization.
National Council on Crime and Delinquency (NCCD) - Nearly a century of experience at consulting and giving expert opinion of criminal justice reform.
National Crime Prevention Council (NCPC) - Home of McGruff the Crime Dog and a number of other crime prevention measures designed to promote greater public awareness about crime safety.
National Criminal Justice Association (NCJA) - A special interest group that represents states on crime control matters.
National Fraud Information Center - A group fighting to increase penalties for telemarketing scams, Internet abuses, and related crimes.
National Center for State Courts (NCSC) - Serves as a clearinghouse for information about modernizing the court, jury, and trial systems.
National Criminal Justice Association (NCJA) - Public interest group that represents the states whenever the federal government considers crime or justice-related legislation.
National Criminal Justice Reference Service (NCJRS) - What's left of the LEAA-LEEP bureaucracy, which from 1969-1978 was the federal government's largest agency in the wars on poverty and crime.
National Institute of Corrections (NIC) - Research arm of the Dept. of Justice that looks at correctional practice and policy.
National Institute of Justice (NIJ) - The research and development arm of the Dept. of Justice.
National Organization of Black Law Enforcement Executives (NOBLE) - Presents a unified voice about issues in the administration of justice and inner-city crime from black police chiefs across the nation.
National Organization for Victim Assistance (NOVA) - Nonprofit organization working for victim's rights and services.
National Rifle Association (NRA) - Lobby group promoting the safer use of guns in our society and fighting gun control efforts of every variety.
National Sheriff's Association - Nonprofit organization dedicated to raising the level of professionalism among those in criminal justice.
New Democratic Network (NDN) - Liberal, center-left think tank in DC which produces commentary and advocacy on topics like immigration, environmentalism, and globalization.
Noblis, Inc.  - Used to be called Mitretek; claims to provide an impartial, objective view of what's needed in the criminal justice system.
Office of International Criminal Justice (OICJ) - Formerly at UI-Chicago and now at Sam Houston; expresses ties to a number of different police and correctional interests internationally.
Office of Justice Programs - Founded in 1984 to evaluate the effectiveness of various programs and promote comprehensive planning.
Office of Juvenile Justice and Delinquency Prevention (OJJDP) - Designed to provide comprehensive planning and control of the American juvenile justice system.
Office of National Drug Control Policy (ONDCP) - The nation's newest Cabinet-level agency, home of the "drug czar" in charge of strategic planning, tells the DEA what to do.
Office for Victims of Crime (OVC) - Nation's leading advocate for the fair treatment of victims, a resource coordination agency.
Open Society Institute: US Justice Fund (Soros Institute) - Funding entity looking for ways to turn crime debates into discussions about ways to improve social life in general.
Partnerships Against Violence Netword (PAVNet) - A group of seven separate federal agencies working together on the problem of violence and the reduction of redundant information.
Police Executive Research Forum (PERF) - Partnership organization of academics and chiefs from the nation's largest cities dedicated to innovative policing, professionalism, research, and involvement in public debate.
Public Agenda - Nonprofit, public opinion and citizen education agency that provides background information and discussion frameworks on many issues where public opinion could be misinterpreted.
Rand Corporation - Nonprofit and national major think tank on policy decisionmaking, involved in a number of national security issues, including crime control, career criminals, drug laws, sentencing, prison management, and equity of criminal justice.
Research Triangle Institute - North Carolina-based think tank that conducts research on crime problem mostly from an environmental or epidemiological standpoint; drug research, and police systems.
The Sentencing Project - Group devoted to alleviating prison overcrowding by advocating alternative sentencing and corrections policy, hosts of the Campaign for an Effective Crime Policy.
Society of Police Futurists International - Partnership group between academics and practitioners which seeks to develop long-range forecasts about crime and the use of technology in crime fighting.
U.S. Census Bureau - Maintains the most commonly uses datafiles that agencies use for planning, etc.
Vera Institute of Justice - Nonprofit agency working to establish more humane alternatives to prison and the justice system in general; their 1961 Manhattan Bail Project introduced the concept of Release on Recognizance, for example.
Violence Policy Center - Nonprofit agency concerned with firearms violence, the number of guns manufactured in the U.S., and anything gun-related.