Conker Live and Reloaded was a great game, but there really aren't a lot of people playing it nowadays. Of course there's Halo 2, Unreal Championship 2 was ok, I think I got mine for $5 from Amazon. If you like fighters there's Capcom Vs. SNK 2, Street Fighter Anniversary Collection, and Guilty Gear X2:R, not sure how many people still play those. Splinter Cell Chaos Theory is one of the few games with online co-op. Rainbow Six was the de facto multiplayer game before Halo 2 came. If you meant offline multiplayer then definitely Fuzion Frenzy, one of the best party games ever, the 2nd one for the 360 pales in comparison, even with its online play. Also, the Lego Star Wars games have 2 player co-op but are said to be short and easy. X-Men Legends 1/2 and Marvel Ultimate Alliance along with Baldur's Gate 2 are all worth playing with a friend, not so much alone.
If you're looking for deep most people recommend Knights of the Old Republic 1 and 2, but I've never played them. And Ninja Gaiden is pretty much a must have for any Xbox owner. I'd recommend Jet Set Radio Future and Tetris Worlds, both were pack-in games with the Xbox and should be cheap (along with SW: Clone Wars and Sega GT 2002 on the same disc). Other not so deep games I still think are worth picking up: Psychonauts for sure, Jade Empire, PoP Sands of Time, Riddick EFBB, and Beyond Good & Evil if you haven't already played them on PC. Most people like Fable and GTA San Andreas, but I personally couldn't get past the first hour or so of either, I thought they were both shit.
Games worth at least a rent: Oddworld Munch's Oddysee and Stranger's Wrath, Timesplitters 2, Otogi 1/2, Stubbs the Zombie, Metal Arms, Incredible Hulk Ultimate Destruction, Black, Warriors, Tony Hawks Underground 2, Shenmue 2, Manhunt, Ultimate Spiderman.