mcMMO is an open source plugin for Minecraft multiplayer servers that is currently being developed by nossr50. 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 obtain the free version from the official spigot page here.
The latest version as of editing is 2.0.0 for Minecraft 1.13.
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.