Bukkit PerWorldPlugins
Aswell as the bukkitdev here: http://dev.bukkit.org/bukkit-mods/perworldplugins/
You are free to examine and modify the code as long as you do not publish this code as your own and if used give full credit to the author (_MylesC).
PerWorldPlugins is built to respect compatibility to as many plugins as possible and reduce chances of incompatibility.