Mobsters is a set of roleplaying rules for 1920s gangsters. It’s a well-done and complete game.
Castles And Crusades is old school Dungeons and Dragons, with some clever systems that both simplify things and make the game compatible with all of yoru old (monster manuals and such. I don’t normally link to commercial products here, but I’m particularly taken with this one.
There’s also a growing community offering free supplementary materials, like the Netbook of Classes. The Netbook offers fifteen new classes, including executioner, harlequin and lore warden.
OSRIC (Old School Reference & Index Compilation) offers a free fantasy role playing game inspired by first edition Dungeons and Dragons. It’s not the first effort I’ve seen trying to recapture the magic of 1st edition, before the life was sucked out of it with the bloated later editions.
The Farland Mass Combat System is designed to help role players resolve large scale conflicts. There’s also a great — and free — program to help you do so.
I recently played this semi-role playing game with my AP Political Science students and we all enjoyed it very much. In Executive Decision, the players each take on the role of a member of the president’s cabinet and try to promote their agenda as critical events unfold in real time. Highly recommended.