Administrative Office of the US Courts--The AO maintains data on the activity of the lower federal courts. 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 US government and links to available data on other government websites.
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.
International Crime Victims Surveys--A "victimization" survey performed in sweeps from 1989-2005, with data from the most recent sweep available and external links to prior surveys. The surveys are also accessible via the NACJD.
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.
National Consortium on Violence Research (NCOVR)--The NCOVR is a multi-disciplinary group devoted to the study of inter-personal violence. The NCOVR's resources include a data center that that provides a web interface to data from the Uniform Crime Reports, the National Incident-Based Reporting System, the National Crime Victimization Survey, and other data collections.
Social Science Electronic Data Library (SSEDL)--The Social Science Electronic Data Library is an archive of over 300 datasets covering a variety of topic areas such as Adolescent Pregnancy, Aging, AIDS/STD's, the American Family, Disability in the US, and Maternal Drug Abuse. The archive is well-indexed and allows variable-level searches. Some of the studies are also available on CD's in the Data Center. The SSEDL is also accessible via Databases at Emory..
Statistical Abstract of the United States--Electronic version of the popular resource. The Statistical Abstract contains a wealth of information on numerous socio-economic and demographic indicators for the US. Historical data tables taken from the Statistical Abstract are available here. There is also a CD version of the Statistical Abstract available in the Data Center.