Timesplitters was a launch game for PS2. An overall decent FPS. Single player was awful but the Multiplayer was great, and more than made up for it. It was of course about time travel.

A sequel came out by the name of Timesplitters 2 in 2002 for PS2, Xbox, and Gamecube. It improved single player, and made multiplayer even better. I really like this game and you can get it on any console for $15 new now.

Timesplitters: Future Perfect is making the single player even better by adding a bunch of paradoxes and situations you'd expect from time travel (like fighting alongsoide your future, battle-scarred self). The multiplayer promises to be excellent, and will have LAN support for all 3 consoles, and online play for the Xbox and PS2 versions.

I should mention the guys behind these TS games did the original GoldenEye 007 on the N64 - one of the best console first-person shooters ever. And it shows.

TS2 website:
http://www.timesplittersgame.com/