Also, Uni-Form is worth checking out as well. Using CSS, the form can easily be styled in one of two common ways. Great way to display a valid XHTML form in a variety of ways without changing your code.
Finally, Ill mention a Prototype Form validation set that I’ve been using for a while on work projects. I need to switch it over to jQuery,
but I’ve been lazy about it. I’ve created quite a few custom validators for it and the new release
makes it even easier to combine and change options on the form fields. I’ve not found something in jQuery that is as robust, yet.