UNITED STATES ARMY MILITARY HISTORY INSTITUTE - Excellent site for researching Civil War Units. If you call or write to them, please be patient, they get many requests everyday and they research each one. Please remember that The Carlisle Army Barracks is full of US Cavalry history, located just north of Gettysburg.
THE UNITED STATES CIVIL WAR CENTER - Their name says it all.
HISTORICAL DATA SYSTEMS INC. - Excellent site for Civil War Database information on Units, Officers, individual soldier records, and statistics.
THE CIVIL WAR ARCHIVE - Source for basic Regimental Histories.
JAMES RIVER PUBLICATIONS - Great source for Civil War books and publications.