When you build a form on the web, you should always validate the data being submitted. While it’s necessary to do it on the server side, it’s more friendly if you can do one on the client side so the user doesn’t have to suffer through multiple attempts to submit.
If you’re good with javascript, you’ve probably already written your own. If not, you might be using the built in ones from Dreamweaver or some other tool, or copied and pasted from a javascript website. And they work. For the most part.
But have you tried qForms? qForms is extremely cool. Just plug it into your forms, set a few lines of code, and you can do anything from simply highlighting those form fields that are incorrect (which in itself is extremely cool) to creating dependency rules (for example, if applying for a loan, choosing the ‘joint’ radio button would make all of the secondary applicant fields required.
Oh… and it’s free. Can’t beat that. They seem to have stopped development work on it about a year ago, but they left a pretty stable product. And there’s an active mailing list as well. It’s definitely worth giving it a shot. And if you like it, toss the developer a couple of bucks in his paypal. It’s worth a lot more than that, for the amount of time it saves.