If you want a really enjoyable and slightly different FPS experience, Star Wars: Republic Commando is excellent.
Dont let the Star Wars tag deter you from it. Its actually a good Star Wars game (yes I know, that suprised me too).
The single player campaign is pretty short. Im not great with single player FPSes usually but I beat this on hard within a week. Great graphics, engine is decent, action at times is majorly hectic. You can imagine charging at a group of Super Battle Droids with your Anti-Armor attachment on your DC-17 and taking down a couple before they incapacitate you, then ordering a commando to revive you and carry on fighting.
Multiplayer is pretty poor. Most kills seem to be via grenade.
Well worth the £16 I paid for it.