The American Battlefield Trust is a nonprofit organization devoted to the preservation of Civil War battlegrounds. They have also created virtual tours of 20 Civil War battlefields.
The NPS website offers a wealth of information about the places, people, and events of the American Civil War.
Unknown author; Kurz & Allison, Art Publishers, Chicago, U.S.A. / Restored by Michel Vuijlsteke [Public domain], via Wikimedia Commons
Ken Lund from Las Vegas, Nevada, USA [CC BY-SA 2.0], via Wikimedia Commons
Waud, Alfred. "Scene of the explosion Saturday July 30th" Library of Congress Prints and Photographs Division. http://hdl.loc.gov/loc.pnp/cph.3g10794
Image from page 342 of "The photographic history of the Civil War : in ten volumes" (1911), via Flickr.
Field works at Franklin, Tenn., occupied by the 23d and 4th corps during engagement of Nov. 30th , Maj. Gen. J. M. Schofield, comdg. [S.l, 1864] Map. .
"Havoc," effects of a 32-pound shell from gun of Second Massachusetts Heavy Artillery. Confederate caisson and eight horses destroyed. Fredericksburg, May 3, 1863. Capt. Andrew J. Russell. Mathew Brady Collection. [Public domain], via Wikimedia Commons
Hospital scene behind the lines of Pickett's Charge, Gettysburg, Pa. Boston Public Library CC BY via Flickr
Kurz & Alison, "Battle of Stone River." Library of Congress [Public domain]
The Dred Scott decision : opinion of Chief Justice Taney, [Public Domain] Library of Congress, via picryl.