Washington and Lee University
Here are several resources which provide systematic coverage of terrorist incidents, groups, etc., with some emphasis on quantitative data. Additional specialized materials are listed here.
"Information on terrorist events around the world since 1970... includes systematic data on international as well as domestic terrorist incidents that have occurred during this time period and now includes almost 80,000 cases." World's largest unclassified terrorism database. From the National Consortium for the Study of Terrorism and Responses to Terror (START), supported by the U.S. Department of Homeland Security and the University of Maryland.
A "comprehensive, searchable and continuously updated database on all available information relating to terrorism, low intensity warfare and ethnic/communal/sectarian strife in South Asia." The site also includes the weekly South Asia Intelligence Review. Maintained by the Institute for Conflict Management (ICP).
"Research and analysis on global terrorist incidents, terrorism-related court cases, and terrorist groups and leaders... over 35 years of terrorism incident data and hundreds of group and leader profiles and trials." From the Memorial Institute for the Prevention of Terrorism (MIPT), which also provides the very useful Terrorism Information Center (TIC ).
The U.S. Counterterrorism Center (NCTC) provides a database of incidents dating from 1994.
Under construction (as of 5/29/07). The International Institute for Counter-Terrorism (ICT) in Israel offers a valuable database of organizations, incidents, and activists, but it has been unavailable for several months. Other materials, such as articles, reports, news, and other publications, continue to be accessible.