The Wolfenstein Series
Wolfenstein 3D
Awesome game for its time. Oldschool to the max. No Multiplayer. id offers the game for download from their site (not for free) via steam.
Spear of Destiny
Sequel to Wolf3D.
Return to Castle Wolfenstein
More up to date game with the same idea. From what I hear the multiplayer is pretty much dead nowadays; most people are playing Enemy Territory now. Requires pretty much the same hack as Q3 and ET for Linux sound support, although there are 2 different scripts (1 for single player, 1 for multiplayer) on the hack's website.
Return to Castle Wolfenstein: Enemy Territory
Based off of the last game, but team based. Available for free from the developers. You'll probably want the et-sdl-sound hack for good sound support on Linux. Multiplayer.