jQuery Validation Plugin
link http://jqueryvalidation.org/
This jQuery plugin makes simple clientside form validation easy, whilst still offering plenty of customization options. It makes a good choice if you’re building something new from scratch, but also when you’re trying to integrate something into an existing application with lots of existing markup. The plugin comes bundled with a useful set of validation methods, including URL and email validation, while providing an API to write your own methods. All bundled methods come with default error messages in english and translations into 37 other languages.
The plugin is written and maintained by Jörn Zaefferer, a member of the jQuery team, lead developer on the jQuery UI team and maintainer of QUnit. It was started back in the early days of jQuery in 2006, and updated and improved since then.