I came across this today when trying to find an easier way to present a six-part form in a Rails project - a tabbed interface system built upon Prototype (an integral part of Rails). Called Control.Tabs it makes tabbed interfaces a synch to do: simply add an unordered list containing links to the blocks containing your to-be-tabbed content, and add a line of JavaScript to start the ball rolling - that's it!

I use the Javascript libraries Prototype and Script.aculo.us a good deal in my projects, but there was one feature they didn't have between them - form validation. A quick google uncovered a super-simple solution to this problem written by an Australian - all you need to start is one Javascript file and one line of Javascript, then add one single DOM-compatible attribute to your input lines and it'll work right out of the box. The even better part is that extending and enhancing it is very easy, just add a CSS line or two to customize your error messages, etc, easy as pie.