The Bureau of Justice Statistics (BJS) is a federal agency under the Department of Justice; that collects and creates creates data, surveys, and reports about inmate populations, prisons, expenditures, deaths in custody, policing, victims of crime, and more. The BJS site is somewhat confusing; this A-Z list shows pages that collocate datasets and reports on given topics, including:
Specific series and data sets of interest for studying trends in populations of incarcerated people, policing, and crime over time include:
the Prisoners series for prison populations
the National Crime Victimization Survey (NCVS); see also Arrest Data Analysis Tool (tables and figures of arrest data from 1980 onward, with national arrest estimates, customized either by age and sex or by age group and race; this tool is produced from underlying data from this NCVS)
the Police-Public Contact Survey (PPCS) - detailed information on the characteristics of a nationally representative sample of persons who had some type of contact with police during the year, including those who contacted the police to report a crime or were pulled over in a traffic stop
The Key Statistics page provides easy access to national trend data. You can also search over available data table files and other files. The BJS is the source of much of the data that is used in reports and visualizations from other sources mentioned in this research guide.
The National Archive of Criminal Justice Data (NACJD) at the University of Michigan archives, curates, and makes available datasets related to crime and justice for secondary analysis. This includes long-term data series such as those published annually by the Bureau of Justice Statistics as well as data from curated studies. Search for data on their site here. The NACJD also produces research guides on complex or frequently-accessed data collections, and you can contact them if you need help with the site.
The National Association of State Budget Officers (NASBO) is the national member organization for state finance officers and publishes reports on state budgets for corrections (among other areas). State Expenditure Reports going back to 1985 are available online.
The Office of Juvenile Justice and Delinquency Prevention is an office of the Department of Justice and creates and disseminates data and reports about juvenile justice issues and incarcerated people under the age of 18. Statistics are available through the Statistical Briefing Book; see the Juveniles in Corrections Overview for FAQs and the Easy Access to the Census of Juveniles in Residential Placement tool for datasets.
The United States Sentencing Commission is an agency in the judicial branch that compiles statistics and reports on federal sentencing, including reports by geography (federal sentencing statistics for each judicial district, judicial circuit, and state as well as the nation as a whole).
The FBI publishes annual sets of statistics on crime through its Uniform Crime Reporting (UCR) Program. They create four annual publications, Crime in the United States, National Incident-Based Reporting System, Law Enforcement Officers Killed and Assaulted, and Hate Crime Statistics.
The Police Data Initiative was started by the White House in 2015 in concert with the Police Foundation (a nonprofit dedicated to "advancing policing through innovation and science") and the Department of Justice. Data from 21 police departments includes figures on use of force, police stops, officer involved shootings, and more.
TRAC (Transactional Records Access Clearinghouse) is a data gathering, research, and distribution organization from Syracuse University; their Immigration Project is additionally funded by various private foundations and seeks to fact-check and synthesize government data about immigration and make it available "in an understandable way." TRAC's Immigration Project provides access to datasets about immigration matters (including ICE Detainers, ICE Removals, detention facility departures. and detention facility transfers) as well as government reports on immigration matters. TRAC also publishes original reports on topics related to immigration.
ICE (U.S. Immigrations and Customs Enforcement), the agency formed in 2002 to merge functions and jurisdictions of various border and revenue enforcement agencies under the Department of Homeland Security, publishes annual statistics on enforcements and removals (these are also included in TRAC's Immigration Project tools). ICE also has a FOIA library containing materials requested via FOIA. These materials are poorly organized but include many key resources on detention facilities and detainees, including raw data requested for large-scale journalistic investigations, reports on specific detainee deaths and a dataset of all detainee deaths from 2003-2017, contracts with private prison companies and other contractors, and detention facility reviews and audits.
The NYC Department of Corrections publishes limited statistics on their website (for more on jails see the BJS Annual Survey of Jails and Census of Jails).
The NYS Department of Criminal Justice compiles data collected by county as part of federal reporting as well as state-level reporting on topics such as arrests, crime, juvenile justice and Raise the Age efforts, drug reform, and other topics. For more NYC data, the NYPD publishes reported crime and offense statistics by city, borough, and precinct on their website, including some "historical" data (back to the year 2000). Some of this data is also available along with some data visualizations via the NYPD's CompStat portal.
World Prison Brief is an open-access resource from the Institute for Criminal Policy Research (ICPR; based at Birkbeck, University of London), which provides access to information about prison systems and populations around the world. Data is collected and updated frequently on topics such as total prison populations, prison population rates per 100,000 of the national population, rates of imprisonment of women, the extent of pre-trial/remand imprisonment, and more. Country pages contain aggregate data as well as links to reports from other sources. World Prison Population Lists are reports aggregating data from countries around the world with trend data.
The Dataset Search tool from Google is still in beta, but promises to "enable users to find datasets stored across thousands of repositories on the Web, making these datasets universally accessible and useful." It is definitely missing a lot of the datasets above as well as many others--use with caution at the moment.