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.

Just a quick mention that Ruby on Rails version 1.1 has just been released, adding some really awesome new features that will continue to make life even easier for web developers everywhere, yes, including even you.