Last week at Remix I demonstrated a markup sanitizer that I’d been working on and announced that it’d soon be available as open source. After a few days and a bit of intellectual property management, I’ve finally managed to get it up on CodePlex under the MS-PL license.
http://markupsanitizer.codeplex.com
The XHTML Markup Sanitizer takes untrusted (X)HTML and massages it into real, trusted XHTML. While plenty of effort goes into preserving the original intent, markup validity and safety is the first priority. It’s particularly useful with content management systems where users are in control of markup, but you want to target XHTML1.1.
The sanitizer does not process entire pages. It is designed to massage snippets of text that users might enter in things like blog comments or product descriptions.
Check it out and try it in your project today! Now you have no excuse for not targeting full standards compliance.
Tatham Oddie 
I haven’t got the chance to try it yet, but how does this compare to HTML Tidy? Apart from the fact that your sanitizer works on snippets and not complete pages – which is perfectly fine.