billiongraves.com
A free site for searching for gravestones and locations of your ancestors' graves. Entries done by volunteers who take photos of graves. GPS coordinates provided.
Find A Grave
Free site for searching for grave sites. You may search and get results without having an account (but they are free). Some entries (all done by volunteers) have resources beyond a picture of the gravestone. Some of these are documented, others not.
Prince George's County Cemeteries
Old website, not being updated, but contains information from Gleanings from the Records of the Francis Gasch's Sons Funeral Home, Prince George's County, Maryland 1860-1940.