jQuery UI is a collection of jQuery plugins and offers great interface widgets that are easy to use. You probably recognize many of them from websites you've used. The only catch is that they don't work with Drupal version 6 without a few tweaks. I had an idea for using the jQuery UI Accordion widget and I had to figure out how to get it working on my D6 website.
There is a jQuery UI module for Drupal (http://drupal.org/project/jquery_ui). Since jQuery UI is a third party library, you have to download it and install it in the module directory. Now there are a few snags... The native jQuery library for D6 is 1.2.6. The Drupal jQuery UI module tells you to download the jQuery UI v. 1.6 library which works with jQuery v. 1.2.6. But, the jQuery UI v. 1.6 is no longer available on the site. jQuery UI v. 1.7 is available, but it requires jQuery 1.3. That left me with a choice: find the obsolete library, or upgrade Drupal's native jQuery library. I chose the latter.