NoEntityGrief (formerly known as NoEnderGrief) is an ultra-lightweight plugin designed to simply stop mobs and entities from messing around with your world.
NoEntityGrief prevents: (Entirely multiworld configurable)
Code was written from scratch by me. One class file and 46 lines in total. (A mere 2.54kb in size!)
BukkitDev's 1000th accepted server mod!