There's a lot of data on criminal justice — prison populations, crime rates, police policies, etc — but it can be hard to find, because it's scattered across and deep within thousands of local sites. Hall of Justice from the Sunlight Foundations is an effort to catalog a significant portion of reports and datasets.
While not comprehensive, Hall of Justice contains nearly 10,000 datasets and research documents from all 50 states, the District of Columbia, U.S. territories and the federal government. The data was collected between September 2014 and October 2015. We have tagged datasets so that users can search across the inventory for broad topics, ranging from death in custody to domestic violence to prison population. The inventory incorporates government as well as academic data.
Dealing with those pesky government PDF files is up to you. At least there's an app for that.