The Civil War Archive has a Regimental Index with Union and Confederate regiment information “Compiled by a Civil War Veteran from The Official Records of the Union and Confederate Armies and a variety of other sources, Dyer’s Compendium offers the most complete list of organizations mustered by the State and Federal Governments for service in the Union Army. However, discrepancies may exist due to a variety of factors, including the sheer volume of records researched and the accuracy of the records used. Although few in number, discrepancies have been found by modern historians. If you have verifiable information that contradicts the official record offered here, please e-mail me and I will present it alongside the existing information.”
This site also has Soldier’s diaries, Soldier’s letters, Battle reports, and more.