John Resig released a screencast about an “Expandable Sidebar Menu”[0], in other words: An accordion menu. While his screencast is a great demonstration and tutorial for everyone beginning with jQuery, the menu itself is somewhat limited. If you are looking for a more flexible implementation, you got to just the right place.
The original implementation of this accordion menu plugin for jQuery is from Frank Marcia and was enhanced a lot since then. It can work with most markup structures to create the menu and will therefore degrade gracefully, in other words: works without Javascript.
Details, demos and documentation can be found on the plugin page[1]. As you can see from the comments there, the plugin is already used and should therefore be pretty bugfree now. Nonetheless, if you spot a bug or need a certain feature, just let me know it.
[0] Expandable Sidebar Menu Screencast on the jQuery Blog
[1] jQuery Accordion menu plugin