Field Validation JS
LiveValidation is a lightweight library that equips you with set of validation functions. Besides the classic validation methods, the library has a powerful live validation feature that controls the fields as you type. Ruby on Rails developers will find it very easy to use because the naming conventions and parameters are very similar. It is a stand-alone library but has a Prototype version, too.
Out of all the js libraries available, I have found this one to be most useful… it is highly extensible, lightweight, and is also is widely supported… Check it out today!