The jQuery form validation plugin is a simple way to validate the HTML forms on your websites. take a look at some of its advantages:

– Open Source project.
– Does not allow double validation code in your site.
– Includes demos and documentation.
– Cross-browser validation and support.
– Dynamic validation.
– Translates error messages.
– Flexible and customizable.

The licence for this validation plugin is MIT, so you can do whatever you want with it; even for commercial purposes.