McMMO is an open source plugin for Minecraft multiplayer servers that is currently being updated by t00thpick1 and TfT_02. McMMO allows for an RPG-like experience to exist in the world of Minecraft complete with skill leveling, special abilities, rare loot, brand new mechanics and a more in-depth PvP! Playing on an McMMO server requires no changes to your client, but servers require Spigot or CraftBukkit to use it. Server owners can either buy the plugin from their spigot pageto help support development or download the free version here.
The latest version as of editing is 1.5.04 for Minecraft 1.9.
McMMO's main function is to incorporate an RPG-like skill system that players will have access to right from the beginning. Depending on how much a certain tool is used or a certain action is performed, players will gain experience that will eventually add bonuses such as double drops, rare drops, or ability enhancement.