FormCheck is an awsome MooTools class that allows you to validate XHTML form fields on cliet side. It has a nice UI touch and the validations are pretty good. I have been using it for some time now but the latest version (at the time of this writing), version 1.6, was released back in March 2010 and it only works with MooTools version 1.2.
I don't use MooTools 1.2 anymore and since there's no FormCheck version released that works with MooTools 1.3, I decided to debug and modify FormCheck v1.6 to work with MooTools 1.3 without compatibility.
You can download the modified version from here.