ModDevKit

DevKit is a mod by Antonio that allows you to create your own mods ingame. The modding files are stored directly in the world data, so they are aviable by everyone, easily changeable via the inbuilt editor and it's easy for everyone to upload his files. (not everyone owns his own server, actually quite few)

It loads all submods directly when loaded, so each Mod or Plugin created with it is loaded completely normally as all others, so there is no limit for those addons.