Doom is available for linux at:
ftp.idsoftware.com/idstuff
Most (if not all) if ID's games are ported to linux (including Doom3 :-))
However, you do have to have an purchased copy of the game since the downloads don't include the graphics, player models, etc.
Also, if you're really interested in linux gaming, check out
www.transgaming.com
They provide compatability for windows games, although I've never used it myself.
Cheers,
Matt
