All Flesh Must Be Eaten (AFMBE) is produced by Eden Studios Inc. and has won multiple Origins Awards.
It uses the Classic variant of Unisystem (unisystem) to represent a gritty and realistic world with lots of attention to the details of combat.
There are many books available to bring zombies into many other genres and settings. This list includes Argh! Thar Be Zombies! for a pirate theme, Dungeons and Zombies for a classic fantasy setting and Enter the Zombie for a Wuxia/Gun-Fu style game.