Halo and Halo 2, I suppose. I never really liked either of them, but my brother really sucks hardcore and I kick his ass...It pisses him off so much <img src=smilies/magbiggrin.gif>
Unreal Championship, Mortal Combat, Marvel VS Capcom 2, Guilty Gear XX or whatever, Sega GT Online, Jet Set Radio Future (Fucking addictive, man).
Thats all I can think of for multiplayer...Get Splinter Cell <img src=smilies/magbiggrin.gif>