Historical

Digital Deadwood
Interactive educational scenario set in Deadwood, South Dakota. Players can wander around the town, learn about the history of the Wild West and play in the casino.
http://www.digitaldeadwood.com/

The Eternal City
An online roleplaying game set in Iridine, a Romanesque city.
http://www.skotos.net/games/eternal-city/

Omerta
Hosts a massively multiplayer mafia game with the action taking place in a crowded city in the 1930's. Offers message board.
http://www.omertamafia.com

Phantasie Crossroads
Dedicated to the preservation of medieval fantasy Free-Form roleplay and writing, as well as the idea and hope of creating a positive online roleplaying community.
http://phantasie-crossroads.tripod.com