Free Form Validation Library: JSValidate

JSValidate is a free library for validating forms that requires Prototype & Script.aculo.us to run.

Some rules it supports are:

Required field
Only numbers
Only letters
E-mail
Credit card number format
US zip code, phone number & state code formats

It works by simply updating class tags of forms with the related rule information.
It lets you define in which tag & class the errors will be displayed & some more.
There is also a jQuery port on the way, named JValidate, which is not out yet but will be sharing at WRD for sure when it is published.

Requirements: Prototype & Script.aculo.us
Compatibility: All Major Browsers (Safari not tested)
Website: http://www.jsvalidate.com/
Demo: http://www.jsvalidate.com/demo/
Download: http://www.jsvalidate.com/download/