Bureau of Justice Statistics
The BJS provides a wealth of crime and criminal justice data compiled by the U.S. government and links to available data on other government websites.
Historical Violence Database
The Historical Violence Database, which is hosted at the Criminal Justice Research Center 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.
National Archive of Criminal Justice Data (NACJD)
The NACJD at the ICPSR contains a wealth of data relevant for scholars interested in crime/criminology. The data cover topics such as offical crime statistics (at the national and state levels), surveys on attitudes towards crime, surveys of crime victims, and the criminal justice system.