MochiKit is a highly documented and well tested, suite of JavaScript libraries that will help you get shit done, fast. We took all the good ideas we could find from our Python, Objective-C, etc. experience and adapted it to the crazy world of JavaScript.
It truely is about doing more by coding less. The async library and it’s use of Deferred is one of the best I’ve seen, once you “get it”, it has a lot of utility. Outstanding design patterns, well documented, test cases and MIT license. I do hope they keep adding more stuff to it with that same level of quality.