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.