This is a great api for getting mobs into your game quickly!
If you have an idea for a mob, this will take care of all the basic function and also has some added functions to easily extend mob behavior! It is well maintained and reported bugs are squashed in a timely way!
One of the few if not only existing author that takes their mods seriously to ensure critical bugs are fixed promptly. If your a serious server admin like me, you can't go wrong with their mods' collection and the fact that prompt support is normally provided to boot. :)