I’ve been playing around with Yahoo’s UI library lately and I’ve got to say, I’m pretty impressed with the improvements made over the last two months. I really dig the YUI animations, very solid replacement for script.aculo.us in my opinion.
My favorites of the bunch would have to be the autocomplete, calendar, and CSS grids.
YUI utilities and controls combined with some solid core PHP framework (maybe Zend, Code Igniter, BlueShoes) really should make for a solid grab bag of tools to rapidly develop web apps.