The Dead Walk is pretty good ( and Free!) and Xtreme Zombie Hunter is a lot of fun aswell.
I've also played Savage Showdown, which is really easy to adapt to Zombie gaming.
Combat Zone and Shockforce are also dead easy ( excuse the pun) to adapt to Zombie gaming aswell.
Zombie Apocalypse by Game Werks is OK , but it's a little "lite" for a production game, although it has got some good ideas ( using markers similar to Space hulk for Zombies until they are spotted).
ATZ is very good for solo play and is pretty easy to learn ( which is a bonus!) and has good support.
As LTD said I'm working on a rule set for Zombie gaming ( damn these things take a long time to get right!!!). I've played a few games and I am ironing out balance issues. The last recorded battle ran really well and the the system coped quite easily with 60+ Zeds on the table.
At the end of the day, most skirmish systems can be adapted for Zombie gaming. The only real thing to address is zombie movement, deployment and reactions. Stat wise they are tough but stupid humans, easliy defeated on their own but lethal in groups!
Regards
PM