PEAR::HTML_QuickForm http://pear.php.net/package/HTML_QuickForm

patForms http://php-tools.de/site.php?&file=patForms/overview.xml

Form Validator (untested) http://www.phpclasses.org/browse/package/1576.html

Validate.PHP for Smarty http://www.phparchitecture.com/script_show.php?id=6