Edit: Forgot to mention GTA VC, and MaxPayne are not multiplayer games, just incase you didn't know, but they are good none the less.
if you like GTA3, GTA Vice City will keep you occupied for a long long long time. I consider myself an excellent gamer, I've beaten Quake3 on nightmare, and played through all the first 2 monkey islands so I have reflexes and problem solving skills, and yet even after about 3 months of playing GTA VC I'm 62% done with it. I beat the main plot about 2 months ago (which left me at about 47% I belive), and I've just been having fun figuring new things out, doing extra missions, and blowing stuff up
Jedi Academy is deffinatly a great game! Its graphics are no where near state of the art, its still on the Quake3 engine, but if you liked outcast but found yourself replying missions like bespin, and others with the dark jedi you'll love this game! Its lightsaber dueling is a LOT better, and its missions make you feel like your really a jedi. It really goes back to the roots, and brings story and gameplay back to the #1 spot instead of graphics as so many games now adays do, and it works splendidily!
Other games I'd suggest.
Savage - Mix RTS and FPS together and you have savage. I haven't played it, but I've been told if you can find good player online it can be a great game!
Max Payne 2 - I didn't list this one above because I've heard its very short. I put in my first hour on it yesterday, and can't wait to get back to it today. Its physics and everything are supuerb. Read gamespys review or actually probably any review, its a nice 9/10 game!
Raven Shield - The 3rd installment of the Rainbow 6 series, raven shield fixes everything that was wrong in the previous games! I quit playing CS for this game, its that good! If you have a good connection and a decent PC this game is increadibly addictive!