Vance Lucas has created a "validation library that dosen't suck" and posted about it to his site - his Valitron library.

Valitron is a simple, minimal and elegant stand-alone PHP validation library with NO dependencies. Valitron uses simple, straightforward validation methods with a focus on readable and concise syntax.

He created the library because of frustration with other tools that had (sometimes major) dependencies on other packages. All he wanted was something lightweight and easy to use that was flexible about how it handled its validation rules. Valitron lets you apply a validation (like "required") against multiple fields in the provided data and includes checks for things like length, valid IP, active URL, alpha-numeric, date format and regular expression match. You can also add custom validation rules via a callback/closure.