jQuery UI, the official user interface components library of jQuery, offers a lot for quickly implementing widely-used widgets and features into websites.
It consists of the popular datepicker, slider, accordion, progress bar, drag 'n' drop and much more.
And, it is themable, there is even ThemeRoller which allows anyone to customize their own themes.
However, any ThemeRoller-customized theme will still smell jQuery UI which can be seen in many sites.
If you are looking for highly-customized (like Bootstrap or Windows-Metro looking), quality and free jQuery UI themes, there are not too much options and here they are:
An awesome theme for the Twitter Bootstrap fans as it transforms the widgets into the Bootstrap style.
A slick, retina-ready jQuery UI theme which makes use of CSS3 gradients and some @2x images.
It works well in both dark and light backgrounds.