Timesplitters 2 - it's a bit old, but it's still the best FPS ever released for PS2. Hands down. The game features awesome single player challenges, deathmatch and story modes as well as 4 player support for each of them. The soundtrack is THE best game soundtrack ever. Each level has it's own music, and it is just incredible. I won't get into the plot, but one level you could be in the 1800's using revolvers, while the next level, youre using laser rifles to chase down a hacker in a far-future Tokyo. This isn't much of a review, just a recommendation. Go rent/buy/'acquire' the game. I guarantee you will find it entertaining and challenging.
Man i love this game. The multiplayer is AWESOME, after you beat the game there are endless amounts of mini-missions and challenges. I still havent beat this game 100% but im still trying.
My friend and I almost beat the story part of the game by co-op, except he deleted his memory card. The hardest part of story is NeoTokyo on hard mode.
I may hate to admit this, as much as I love my XBOX, but this is one of the few shooters that is actually better on PS2 than XBOX.