I think it's safe to say that my favorite game of 2009 is Left 4 Dead. Jodi got it for me for Christmas, and I've been playing it ever since. It's got zombies, it's a first-person shooter, and it runs on Valve's Source Engine, which is the best engine I have seen to-date. All are wins in my department. I'm a huge fan of most of Valve's big games, and this was surely no exception.
Right now Left 4 Dead only has four campaign maps and a versus mode that you can play. I was excited to hear recently that they were planning on releasing new maps and game modes in the coming months. Rumor has it they're coming out with a "Survival Mode" where your group gets a few minutes to hunker down before being assailed with a massive onslaught of zombies. If it's anything like what we're doing now with the mob level hacks set to insane, it should be a blast.
I was looking for any information about the new maps, and thus far I haven't found much. I did find what appears to be a third-party map where this guy put together a pretty good replica of the Crossroads Mall from the Dawn of the Dead movie remake. But I can't seem to find the actual maps that are being released by Valve. I did, however come across a couple of fun L4D-related items in my travels. First, an advertisement for Left 4 Dead hosting that I thought was pretty funny (it's not as funny if you don't play the game). Click the image for a larger view:
And then there's this t-shirt from Ctr+Alt+Del called "Cocktail Party" which I got a good laugh out of: (as before, click the image for a larger view)

Good stuff, good stuff.