I stumbled across Vanadium while reading I believe Ajaxian's blog. Vanadium is an excellent client side form validator for the jQuery framework. Check it out.
Here is some sample syntax. Say you wanted to verify that a user entered in at least four characters in a textbox.
<input class=":max_length;4" type="text">
Vanadium uses the class HTML attribute to set the validation rule. Many rules are supported.