jQuery UI is a collection of widgets based on jQuery. Made by the same group that develops jQuery.
I use it mostly for its datepicker widget. Adding the widget is as simple as attaching datepicker to a form element, and jQuery UI does the works.
Recently I also used its autocomplete widget.
It makes things a lot easier.
And, it also comes with its own set of themes that you can use to fit your website colors.
Now, what about you ? Have you use jQuery UI ?