Okay, so some people may have seen this in the Zombies topic below, but I think it deserves it's own topic.
I'm gonna try and keep this short, I tend to ramble on + on. But anyway essentially the game is a sandbox zombie survival, running off a slightly humerous (for me anyway) story, and has you running round the city basically trying to survive. You can do as you wish, take over the many buildings with survivors in, explore the city and all the connecting parts or just generally shoot zombies into pieces. It has a small RPG like element in the Skills, which you can buy in the different safe houses. There are storyline missions, but you don't have to do them unless you want to rush to the end of the game (I'm 1/3 through it in terms of missions).
There are quite a few weapons, and a few varieties of zombie, but the best part has to be the factions, take out too many SWAT / rebel members and they will turn on you, cutting you down at every step (I tried a rebel character, I got smushed into paste by a SWAT team.
There are three control schemes, Relative, which is turning, Absolute which is isometric, making up go up-right, and Absolute with the mouse. I personally use Relative, it's the easiest sceme.
Anyway, it can be downloaded at
http://skasoftware.com/ , and for good measure have a screenshot of my character:

I have more skills than that, mostly Passive ones, but my fave 3 are Sentry Gun (Fck yeah), Headhunter (15% Instant Headshot) and Regeneration (Restores HP over time, who doesn't want that?)
And that green thing is a flamethrower, and it bloody OWNS!
So if you've played it, good on you. If you haven't, play it, now. OR I'll send a rabid dog round to get you! If you want the story just go to Wikipedia and search for it.