ValidForm Builder is an impressive open source form creation library that relies on PHP and JavaScript (jQuery).

The library outputs standards-based web forms (XHTML 1.0 valid) that are validated both on the client-side and the server-side.

It supports every input type (textbox, checkbox, radio, etc.) and comes with ready-to-use validation rules (integer, required, e-mail, etc.).

ValidForm Builder also has a captcha class which is very easy to integrate and can display a new captcha on every click.