Correlates of War (COW) Project
The Correlates of War Project is hosted at Penn State University and distributes a variety of datasets relevant for the study of inter- and intra-state conflict.
Minorities at Risk (MAR) Project
The MAR Project is affiliated with the Integrated Network for Societal Conflict Research (INSCR) and the Center for International Development and Conflict Management (CIDCM at the University of Maryland. The project tracks the status of nearly 300 "politically-active communal groups" and the MAR data are used extensively by scholars of ethnic conflict. See https://cidcm.umd.edu/research/minorities-risk-project for updated work on the MAR project.
Paul Hensel's International Relations Data Site
This site is run by a professor at the University of North Texas and is a good source for data on international conflicts, economic relations, and organization.
The Uppsala Conflict Data Program (UCDP)
The Uppsala Conflict Data Program is run by scholars at Uppsala University in Sweden. Their catalog is a very comprehensive introduction to datasets and other resources pertaining to international and internal conflict. The UCDP also hosts a variety of data collections on political violence, primarily focusing on inter-state conflicts, intra-state conflicts, and terrorism. Among its collections are geo-referenced data on individual events of political violence. The program's various collections of data are freely available to download. See https://www.prio.org/Data/Armed-Conflict/UCDP-PRIO/ for earlier versions of the data.