There are so many first-person shooters out there that's it can be hard to decide what to play. The following is a list of the five best out there that no self respecting FPS gamer should be without.
1. Team Fortress 2 (PC/Mac): Valve's 2007 team-based sequel is by far the best FPS value on the PC. I say the PC because unfortunately all of their great free updates (Maps, Weapons, Modes) do not make it over to the Xbox 360. That said, you can't find a more fun and varied FPS experience on any platform. As with all Valve games, it will also be available for the Mac very shortly.
2. Left 4 Dead 1 and 2 (PC/Xbox 360/Mac): Unlike Team Fortress 2, the console does see the updates for Left 4 Dead. I included both games here as it's really a matter of preference. The sequel features melee weapons and a New Orleans setting, while the original has a more traditional zombie apocalypse setting.
3. Battlefield: Bad Company 2 (PC/Xbox 360/PS3): The single player may leave a lot to be desired, but DICE's sequel is the best modern-combat themed multiplayer you can find on either the PC or consoles.
4. Quake Live (PC/Mac): An oldie, but a goodie. It may be eleven years old, but Quake III set the standard for the modern FPS. Quake Live takes the core Quake III game and puts it online to play for free. The browser-based game works on both the PC and Mac and has an extremely competitive community.
5. Borderlands (PC/Xbox 360/PS3): Take a FPS and cross it with the loot-based gameplay of action RPGs such as Diablo and you have one of the most unique co-op games on the market. Gearbox has done an excellent job expanding Borderlands with new maps and loot, and they promise to continue that in the future.