HTML Purifier, input sanitization against XSS

http://htmlpurifier.org/

HTML Purifier is a standards-compliant HTML filter library written inPHP. HTML Purifier will not only remove all malicious code (better known as XSS) with a thoroughly audited, secure yet permissive whitelist, it will also make sure your documents are standards compliant, something only achievable with a comprehensive knowledge of W3C's specifications.