<p>Not wanting to re-invent the wheel or anything, I was wondering if there's a plugin out there a field or form validation plugin that works with Prototype that you can recommend from your own experience and why?</p>

I like Ben Keen's Really Simple Validation component.
Features I like:
All the validation rules can be kept within javascript so you're not adding class="required" to your HTML code.
You can display error messages in a javascript alert box or with HTML text.
Adding your own extensions is easy.
http://www.benjaminkeen.com/software/rsv/
Note that I've only used the jQuery version, but a Prototype version is also available.