One late addition: the Stench Zombie
This is basically an attractor monster, similar to "boomers" from the Left4Dead video game or my own Screechers. The Stench zombie attracts more undead to the location of the players. The difference between a Stench and a Screecher is that the latter uses its shrieking call to attract the ghouls to its vicinity; the Stench zombie, on the other hand, leaves a foul slime on anyone it attacks that makes its victim a more noticeable target. This doesn't mean that all nearby walkers will make a bee-line for the unfortunate character, but it does make that character more apparent to the ghouls; basically, it gives the zombies a +2 to all "Perception" checks they make against the player.
The smell wears off after a few hours, but by then it may be too late.