Control.Tabs is a simply wonderful JavaScript extension for the Prototype library. In one project I'm using in multiple places and it fits the requirements perfectly. There is, however, a weird problem with one page with IE where instead of displaying the form like this:

<cut>

Firefox has Firebug, the best JavaScript debugger on the face of the planet. The developers came up with a subset of Firebug that works on all web browers, creatively named Firebug Lite and should be an essential part of any web developers toolkit.