For those others of you hoping to see something BattleTech: http://www.youtube.com/watch?v=nJU4iA4TWIs, http://www.mwlldownload.com/

Mootools allows for using traditional OOP concepts like "Class", "Implement", and Extend" in your JavaScript code. I haven't seem a similar mechanism in any other "lightweight" javascript library, including jQuery. I can think the same in my Java and JavaScript, while simplifying my code base by using inheritance and polymorphism. Having OOP in JavaScript might seem silly or unnecessary, but for large apps, it really helps. When I was coding a website that required drag and drop with validation that the input source could be dropped onto the drop target it saved me lots of time.