From jQuery JavaScript Library

People's use of jQuery serves as good demonstration of its abilities. You may also want to try introductory examples, tutorials or plugins.

Web page functionality demos:

• Stylesheet switching
• Sliding menu
• InnerFade

More complex and/or shinier demos:

• Image Gallery using jQuery, Interface & Reflections
• Chat
• Tetris
• jQuery puzzle
• Google map integration

Incidentally, note that you can use jQuery from the FireBug console on pages that use jQuery - might be interesting for a quick live test.