There's a new GURPS ebook out called GURPS Zombies: Day One for those who are into both GURPS and zombies (like, cough, me). This is a very good complimentary tome to GURPS Zombies, including eight zombie worlds with details on the "day one" starting point for each campaign and taking the setting through to a good finale, with tons of suggestions and material in between. Despite being only 56 pages it does a great job of setting each world up; fans of All Flesh Must Be Eaten will be comfortable with the style taken here....and anyone with an interest in zombies as a subject of RPGs will likely notice that this is a very common style of approach to the genre, since campaigns in zombie worlds tend to be bleak and short, and rarely recurrent in the fashion of more conventional RPG worlds.
Even if you don't want to use GURPS for some reason this is a good setting/concept book with more generic material than rules, but if you do use it with GURPS you'll probably get the most bang for your buck. Check it out.