LiveValidation 
Posted on September 14, 2007
Validation As You Type
http://www.livevalidation.com/
LiveValidation is a small open source javascript library built for giving users real-time validation information as they fill out forms. Not only that, but it serves as a sophisticated validation library for any validations you need to make elsewhere in your javascript, it is not just limited to form fields.
The naming conventions and parameters of the validations are similar to those found in the Ruby on Rails framework, and as such is the perfect client-side companion. Don’t worry if you dont use Ruby on Rails though, LiveValidation can be used anywhere you like, is simple to learn, and a joy to use.