Borderlands, a game that varies greatly from single player to multiplayer
The gameplay is similar to any FPS, just point and shoot nothing really to it at all. The campaign is best played co-op as having a friend makes it a whole lot easier when you are overwhelmed with enemies like zombies or skags. The weapons have the absolute best idea i have ever seen in a shooter, randomising parts and abilities to guns and creating something from this in game random generator, this makes for the most unique gun system i have ever seen in a game. This system can give you some really great weapons and the best of these has to be the shotgun i found on my travels that shoots rockets instead of buck-shot.
The mission variety is absolutely great you will go from getting a prosthetic leg back from a monsters stomach, to fighting skagzilla, to dynamite fishing. Simply put this is one of my favourite FPS' ever.