I had to do some simple form validation the other day and thought to myself that, in our current Web 2.0 world, I should do it with AJAX since it is really everywhere and it didn't even cross my mind todo it the old fashioned way with plain Javascript, which might do the job just a good. 

So, I stumbled upon Live Validation which offers a lightweight, open source javascript library to do just that: realtime form validation with some very useful features. They have some excellent examples here and the code is decently commented. Definitively a must have in my toolbox!

Read more: http://www.livevalidation.com/