The links on the left bar are broken (in Firefox and IE at least). Start here http://jqueryui.com/docs/Getting_Started. Click on any links on the left nav (if you find the links work, click on any under the "Theming" section to produce the error). Once you produce the error, it is impossible to use any links on the left nav, even if you refresh and go back to the original URL given above. Here is the error:

Message: 'parent().effect(...).offset().top' is null or not an object

Line: 107

Char: 2

URI: http://static.jquery.com/ui/js/demos_effects.js