Really Simple Validation (RSV)
This powerful little script lets you add client-side validation to any webform with very little work. It includes a number of pre-existing routines for common tasks like validating email addresses, numbers, and other field content, and provides a simple mechanism to extend it to whatever custom functions you need. For documentation and examples, please visit:
There are three flavours:
/standalone- contains a self-contained script with no other dependencies.
/jquery- RSV as a jQuery plugin
/prototype- RSV as a Prototype plugin
As mentioned this is an oooolllld script (~10 years or more), but still functions perfectly well. If anyone wants to take over the project, go for it. The world is pretty much inundated with validation libraries these days, so I don't plan on spending any more time on it, other than for bug fixes.