For fans of Forgotten Realms, GOG.com has got a whole bunch of D&D games available:
Collection One: Eye of the Beholder I, II and III ($13.79)
Collection Two: Pool of Radiance, Hillsfar, Curse of the Azure Bonds, Gateway to the Savage Frontier, Pools of Darkness, Secret of the Silver Blades, Treasures of the Savage Frontier, and D&D: Unlimited Adventures $(13.79).
Collection Three: Dungeon Hack and Menzoberranzan ($8.29).