If you don't have a GOG.com account, now is a good time to set one up. Unreal Gold, recently covered by LGR, is a classic FPS with large atmopheric levels and great combat mechanics. It was the first game I ever played which had built in bots for playing deathmatch multiplayer against the PC, which was very useful back before broadband internet.
Head over to GOG.com and secure a copy now.
EDIT: It is also currently free on Steam!