Administrative Office of the US Courts--The AO maintains data on the activity of District and Appeals courts in terms of caseloads covering civil matters, criminal matters, bankruptcies, and other areas. Coverage is generally from 1988-present, but extends further back in some cases. These data are available here in .pdf format.
Bureau of Justice Statistics--The BJS provides a wealth of crime and criminal justice data compiled by the U.S. government via a variety of data-collection programs. The BJS also provides various tools to produce and download tables on topics such as crime rates, crime victimization, and corrections populations. Many of the BJS' data collections are available via the National Archive of Criminal Justice Data.
Center for Empirical Research in the Law (CERL)--CERL, which is hosted at Washington University's School of Law, "supports the application of sophisticated empirical methodology to legal studies research." Its various projects cover topics such as judicial ideology measures, Supreme Court opinions, and even a legal encyclopedia of popular music.
Congressional Quarterly Supreme Court Collection--The CQ Supreme Court Collection is part of the CQ Electronic Library in Databases at Emory. It contains a variety of data on the United States Supreme Court regarding opinion activity, decision dynamics, nominations and justice ideology, among other topics.
Court Statistics Project (CSP)--The CSP (hosted by the National Center for State Courts) collects data on the work of state trial and appellate courts. The CSP produces various statistical resources on caseloads of state courts, including breakdowns by categories of cases (e.g. civil cases, felonies). More detailed data files from the CSP are accessible from the NACJD's State Court Statistics Series.
Federal Bureau of Investigation--The FBI maintains a number of reports on crime and arrests collected via the Uniform Crime Reports data collection program. UCR data are available here in .pdf and .xls formats. The NACJD has an extensive guide for understanding, accessing, and using the various data collected via the Uniform Crime Reports.
Historical Violence Database--The Historical Violence Database, which is hosted at the Center for Criminal Justice Research at Ohio State University, is (to quote the website) "a collaborative research project on the history of violent crime, violent death, and collective violence." Data from the project are available for the United States and other countries.
International Crime Victims Surveys (ICVS)--A "victimization" survey performed in different countries in sweeps from 1989 onwards in which respondents are asked about their experiences with crime and with policing and about their feelings of personal safety. Different waves of the ICVS are available in various locations: the United Nations Interregional Crime and Justice Research Institute, Data Archiving and Networked Services in the Netherlands, the University of Lausanne, the National Archive of Criminal Justice Data, and openICPSR.
Judicial Elections Data Initiative (JEDI)--The JEDI ( ... ) project at Washington University's School of Law is devoted to collecting "data on elections to state courts of last resort from 1990 to 2010 in an effort to facilitate replication efforts and to stimulate new research in the area of state judicial selection."
Judicial Research Initiative (JRI)--The Judicial Research Initiative at the University of South Carolina has brought together multiple datasets that are of interest to students of judidical politics, covering topics such as attributes of court cases and justices at various levels of government and measures of judicial ideology. Harold J. Spaeth's data on the Supreme Court may be of particular interest. The Spaeth Supreme Court data also reside at http://supremecourtdatabase.org/.
National Archive of Criminal Justice Data (NACJD)--The NACJD provides online access to and analysis of crime and justice data from federal and state agencies. The data cover topics such as attitudes towards crime, crime stats from official agencies, and the functioning of the criminal justice system.
Office of Juvenile Justice and Delinquency Prevention --The OJJDP offers both its database of National Juvenile Justice Data Sets and Data Analysis Tools as means of access to its various data collections, with a particular focus on juvenile activity in the justice system. The Statistical Briefing Book provides an overview of data collected by or related to the OJJDP. Many datasets funded by the OJJDP are also available via the NACJD.
Sourcebook of Criminal Justice Statistics--The Sourcebook compiles data from more than 100 US criminal justice sources. Available in spreadsheet or .pdf formats.
Statistical Abstract of the United States--The Statistical Abstract contained a wealth of information on numerous socio-economic and demographic indicators for the United States. Historical data tables taken from the Statistical Abstract are available via the Internet Archive. While the Census Bureau has discontinued publication of the Statistical Abstract, the Abstract is now being published by ProQuest and is now available via Databases at Emory.
Uniform Crime Reporting (UCR) Program Data: Arrests by Age, Sex, and Race, 1980-2016--Annual data for individual police agencies for counts of arrests for individual categories of crime broken down by age, race, and sex. The data were compiled by Jacob Kaplan at the University of Pennsylvania.
Uniform Crime Reporting Program Data: Offenses Known and Clearances by Arrest, 1960-2016--Monthly and yearly data for individual police agencies on reported offenses and cleared offenses. The data were compiled by Jacob Kaplan at the University of Pennsylvania.
United Nations Surveys on Crime Trends and the Operations of Criminal Justice Systems--The UN Office on Drugs and Crime (UNODC) makes available the results of global surveys on crime and criminal justice that it has conducted since 1970. You can access results from the early 2000s onwards via UNODC Statistics Online. The NACJD also distributes data from many waves of this survey. The UNODC also publishes statistics on international crime in its various research publications.