I like a lot of the js frameworks out there, but for the 'quick and dirty' (and clean) UI widget dance, I'd recommend:
script.aculo.us. Dojo's also good, Some of the stuff built on top of jQuery's great, but as stated, for pure simplicity script.aculo.us is probably the best. It has a lot of nice features that Thomas Fuchs put in.
Caveat... If you're already using jQuery you'll have to be very careful as script.aculo.us extends 'prototype' and there can be namespace collisions.
Fortunately, jQuery has a "no conflict" mode. (noConflict())