Left 4 Dead, it's kinda neat (at least the offline, single player version), but kinda the same all the time. There's no real story in it, no difficulty curve whatsoever, no awesome new weapons, and it's pretty obvious it's meant to be an online game. But still, the concept and style is pretty awesome. I've spent quite some hours on the slash an 'unofficial' prequel / inspiration, a zombie mod for Counter-Strike Source, and I like how they've developed that into a full game.