jQuery is most popular JavaScript library definitely (see the tag list on your right hand side). However, the "official" UI library, jQuery UI, provides far less widgets than some other libraries (extjs, dojo/dijit, YUI for examples) out-of-box.
Are there any other UI libraries built on jQuery compare those "other libraries"? Preferably free for non open source projects.
Ext JS is easily the most powerful.. another advantage of Ext JS is that it can be used along with jQuery and Prototype... so you can have the best of both. Plus ext js has now redesigned it's architecture to a more MVC style framework, so its more flexible.. BUT it's license is at best dodgy.. so be sure to go through the licensing before you decide to use Ext JS.
Check out the comparisons at wiki