SimpleLoot!
The main use for this plugin is to add emeralds to mob drops, however you can change the item, quantity, drop rate, and even which mobs drop at all in the config.yml. I've also added an option for adding drops to tall grass blocks.
Usage
The plugin uses the config.yml to decide what to drop and how frequently to drop it. for example
CaveSpidersDropLoot: True // Set this to false to stop any special drops from this mobtype. CaveSpiderLoot: 388 //ItemID of the loot you want dropped. CaveSpiderLootQTY: 1 //How many you want it to drop. CaveSpiderLootChance: 100 //the percent chance it will drop.
this means that a cave spider will always drop 1 emerald in addition to what ever regular drops they'd normally have.

