Serious Sam, which can be found for $19.99 new and about $12 used at many locations. It's a fun mindless FPS that offers a good 48 (? I can't remember how many) missions that will take u a good while. Multiplayer is pretty mindless as well, and even someone with no FPS skills can jump into the game. It has LAN CO-OP(yes you read that right) one of the only three things Halo was missing (Live but it was too early, and the other being Bots for multiplayer) and of course LAn death match. It's still missing bots but that's my only real complaint outside of no Live.
Wolfenstein offers two-player co-op single player, and I have no idea what it has to offer for system link. I found the single player to be pretty boring.
Time Splitters 2 has bots, a map editior and system link fun. The problem is I could never get use to aiming my weapon, it just felt real odd.
XII is a decent FPS with lan comptability and bots. Worth the $10 I paid for it atleast.
You know for a system with alot of good FPS, I certainly don't own that many. I'm going to look into picking up Counterstrike eventually but I heard it's worthless without Live. I'm also gonna pick up Red Dead Revolver eventually. I use to own Unreal Championship but I thought it was pretty dull, and I never touched Unreal 2.