Introduction to jQuery UI
According to the official site:
I only ran into it because of an update to WP-Click-Track and Wordpress uses jQuery so I kinda had to. Not that it's a bad thing; I like jQuery a little bit more because of jQuery UI.
There's also a Theme Builder that gives a good overview of what's available by default along with some preset themes to test; there's a good deal of widgets there to work with. Almost, almost, makes me want to start a project with it.
Implementation is really elegant too; if you know how jQuery works it'll be pretty obvious. Otherwise take a look at the below for creating tabs:
This, of course, barely scratched the surface of jQuery UI but it doesn't get much more difficult. If you're using jQuery for a library and you need to make something cool it's definitely worth checking out.