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 UIwhich 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:
jQuery UI Bootstrap
An awesome theme for the Twitter Bootstrap fans as it transforms the widgets into the Bootstrap style.
Delta
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.
jQUIT Builder (Windows Metro Style)
This is a ThemeRoller like theme generator for creating Windows Metro style jQuery UI themes.
Just choose the colors and get the theme built automatically.
Selene
Selene is a theme with light colors and includes styles for all the jQuery UI widgets.
Aristo
This is Cappuccino’s Aristo theme ported to jQuery UI.
And, PSD files for Aristo also available for free.
Absolution
A jQuery Mobile-inspired theme with a clean look and making use of CSS3.
The theme is specially handy for anyone looking to catch the same feeling between (jQuery) mobile and desktop.