My good friend Massimo Foti has come out with a new JavaScript form validator that will knock your socks off. It does just about anything you can possibly need in a validation script, including filters (stripping characters as you type), custom error messages before submitting, upload validation, and image dimension checking, in addition to the standard validation features for almost any type of user input. The best thing about it is that Massimo's code is always well documented and well thought out so that it is easily integrated into your site and easily customized. Also, it's a free download from Massimo's site:

http://www.massimocorner.com/validator/

See it in action here:

http://www.massimocorner.com/validator/samples/