CivilWar.com
This site is very broad, not deep enough to be of much interest, except for the indexed links page. Pretty, though.

Library of Congress Photo collections
Well over a thousand photographs in a couple of collections, indexed and searchable with descriptions and provenance.

Treasurenet
This site has a collection of several hundred ACW photographs.

Fire and Fury American Civil War Rules Site
Contains scenarios, rules modifications, clarifications, and discussion. There are also some random unit and scenario generators for those who need inspiration in setting up a game.