I recently started looking into Javascript libraries such as Yahoo! UI (Y! UI), jQuery, Prototype, Mootools, Dojo etc. and have come to like the first two, primarily due to the quality of documentation they have. The latter two are also very popular but aren't (in my humble opinion) libraries that a Javascript beginner can get into. There is very little documentation in terms of tutorials or example pages and frankly I was pretty overwhelmed by them.

Y! UI by far has the best documentation of the lot and has pages upon pages of examples. In fact, the documentation is so good that it made the library seem a little complicated at first. However, once I took the time to read portions of it carefully, I found it to be excellent. There's a lot of customization that can be done to the provided widgets and a lot of work is being done by Yahoo!'s developers to get new features out.

However, my library of choice (at the moment) is jQuery. The main thing that drew me to it was its syntax. I found it to be very similar to CSS, which I also like working with. I don't want to get into it very much now, but if you'd like to know more, check out jQuery and Learning jQuery. In a recent posting on the Learning jQuery blog, more tutorial resources were provided so you might also want to give those sites a look-see too:

Tutorials Elsewhere (Via Learning jQuery)