BF1942 is my personal all-time favorite, beating BF2 by about 1 point on a 10 point scale.
Some others that I find exceptionally good are System Shock 2 (which I've been playing lately with a friend), Raven Shield, Quake 2, UT2004 (especially the Deathball mod), Half-Life series and mods, and There.
I feel like throwing in a quick description of System Shock 2 since most of you probably don't know what this great game is about.
It has a storyline that seems to blend Half-Life and the Alien movie series, and the "I, Robot" movie has me convinced that they got some ideas from System Shock. It is considered one of the best FPSRPG's and probably the first to do a good job at it. The graphics are still very good considering it was released in 1999, especially with the third-party Rebirth model/texture upgrade. Multiplayer, which is very enjoyable, is co-op only supporting up to 4 players, and voice chat (with third-party software) is mandatory because there's so much communication required that text chat (built in of course) would be too much hassle. I estimate a good 20 hours
at least to play through the whole game at a normal rate (which involves making sure you don't miss anything), although it seems to do a good job at making it feel much longer with the large levels and many objectives.
More information:
http://en.wikipedia.org/wiki/System_Shock_2
Download:
http://www.the-underdogs.info/game.php?gameid=3924