Well here we go
One XboX with multi-player
Halo 1 & 2
Fuzion Frenzy
Kung Fu Chaos(Personal Favorite)
Street Fighter Aniversery Edition
Trivial Pursuit
All Tony Hawk Games
All Burnout Games
Cel Damage(you can get this cheap)
Fight Night 1 and 2 (boxing game)
Lego Starwars(Best star wars game ever)
More Then One XboX Multi-Player
Halo 1 & 2
Midnight Club 3 (really fun racing game)
Ghost Recon 2 (the more people playing, the better)
Splinter Cell 3 (4 people max)
Rainbox 6:Black Arrow
Counter Strike (you can get this title pretty cheap)
Crimson Skies(you can get this one cheap also)
Shattered Union (good real time strategy game)
Full Spectrum Warrior (2 players, but fun war game)
Star Wars Battlefront 1 & 2 (haloish but more thinking and more option)
Star Wars Republic Commando(haloish)
MechAssault 2 (haloish, but with machines and alot more option and bigger maps)
Single Player
Ninja Gaiden (best game on xbox)
Red Dead Revolver (fun old western story game)
Breakdown (hard as hell, good storyline)
Riddick (probably the 2nd best game on xbox, even tho the movie sucked)
Halo 1 & 2
Midnight Club 3
Splinter Cell 1,2 & 3
Destroy All Human(little redundant but fun)
Def Jams Fight For NY(fighting game kinda like Street Fighter)
Doom 3(best to play alone at night, with good surrond sound)
Full Spectrum Warrior
Mercenaries(Grand Theft Auto, but with the millitary)
This is all depending on personal taste, pretty much every game listed is worth the cost of which it is sold. I'll add more if i can think of some.