I always thought the Imperial City Sewers were really cool, but running around bashing mudcrabs and rats gets boring really quickly.
This mod adds two types of leveled zombies to the Imperial Sewers, Sewer Zombies and Ancient Sewer Zombies. The Ancient Sewer Zombies are tougher than the normal ones and they glow in the dark. The zombies added by this mod are friendly towards undead and vampires but hostile to creatures, goblins and bandits. So this can make the sewers a bit more interesting.
This mod does not add zombies to the Old Way, or the parts of the Elven Gardens sewers that are important to the Mythic Dawn questline, the "Sunken Sewers".
It does add one Sewer Zombie to the prison sewer that's part of the tutorial.
A word of warning, if you exit the sewers while any zombies detect you, they will follow you out of the sewers. The guards take care of them quickly enough, but just thought I should let you know. :)

EDIT: As of the 1.1 update the sewer zombies are no longer leveled. The 1.2 update removes edits to vanilla factions which in hindsight were completely unnecessary and adds the Sewer Zombies to the Undead faction.

Compatibility
This mod adds new spawns to the sewers and removes a few spawn points as well from the vanilla game, replacing them with the zombie spawner.
This mod has been cleaned with Tes4EditQuickAutoClean.

Install
In the rar file, you'll find a file named "SewerZombies.esp". Drop the "SewerZombies.esp" file into your Oblivion/Data folder. After that, in the Oblivion launcher, click on the "Data Files" tab. In the list there, check the box next to "SewerZombies.esp". Now you're good to go!