One of my biggest concerns when building a dynamic website is, “How do I validate the users input?”
There is a myriad of different front end scripts that have popped up to handle the front end/live validation, one that I started using was LiveValidation, It was a useful library, but Since I was already using MooTools anyway, there was additional overhead to use this additional library, as well as the validation messages all appeared inline with your other content, and this tended to be a difficult thing to work with in cramped conditions.
This led me to search around a bit more and I found Formcheck, A live validation script based on mootools. It was lighter weight, since it was using the library that I was already using. The configuration/integration was about 100 times easier to implement and manage. And the notification appeared as a hovering box rather than and inline notification, which was just the icing on the cake.
Light, Easy, and Pretty, who could ask for more.