Wijmo

I just found out about Wijmo. Wijmo is a javascript widget library that utilizes jQuery, jQuery UI and knockoutjs. Using jQuery UI allows the widgets to be themed easily with any of Wijmo’s premium themes, the canned jQuery UI themes, or rolling your own with theme roller.

Wijmo costs if you want to use it in a commercial application, but they do have an open source option that allows for use in GPLv3 projects.