I like Zend Framework's Zend_Filter class. It's basically a set of methods for validating untrusted data. Although the two arguably most important features isEmail() and isUri() (the latter can be worked around with Zend_Uri) are still missing, the whole thing looks promising already.
He starts with a few of his thoughts on the package ("Clean up the code of isHostname", "International support for isPhone") and some code to show how it can be implemented - filtering for use in a regular expression, validation of a string by a character whitelist as well as blacklisting, and using the Zend_Filter module for a more flexible isName.
