MobTimer gives mobs (or any spawnable entity... I think) a configurable lifespan. When their life expires, they will either die or vanish, depending on the configuration. Lifespans can be configured per-entity and per-source (That is, you can have zombies only live for 5 minutes, but zombies created by spawners only live for 30 seconds).
Feature overview:
This is a simple mod I made for the server I play on to deal with the thousands of mobs created by the various EXP farms setup across the world. It works great for us, but it may not work so well for everyone or every server. Your mileage may vary, etc., etc.
Feel free to use or modify this mod as you see fit. All I ask is for proper credit for any derivative projects (or maybe a nice donation :D - click the "donate" button on the upper right).
-C
