[avital]

MooTools (http://mootools.net) contains all the capabilities of jQuery and Prototype.js, together with a very impressive class system which allows for better modularity in your code. It has a very supportive community and is being constantly developed. You might also want to read Aaron Newton's long post comparing jQuery to MooTools: http://jqueryvsmootools.com/

That being said, you'll find much more people fluent in jQuery or Prototype.js than in MooTools.

[mistermann]

Yes, Aaron's post is a very good read. I still went with jQuery nonetheless.