Every once in a while, a sexy tool comes along that makes you go "Now why hasn't anyone thought of this before?" Ben Hammersley's XHTML Validator to RSS is one such example.
In a nutshell, it removes the tiresome job of validating your page with the W3C's XHTML Validator whenever you make a change. Instead, you subscribe to an RSS feed that points the web page you're working on at the validator, and then whenever you update the feed, it returns all the errors and warnings as RSS items. Nice idea.