Oh, this is very handy indeed: a quick lint tool for HTTP so you can see what kind of headers are being sent. There’s a bookmarklet in the footer too.
Nicholas and Nicole have unveiled the CSS companion to JS Lint. And yes, it will your hurt your feelings.
Finally! A lint tool for HTML (including HTML5) so you can enforce strictness in your writing style.
A lint tool for microformat values on the rel attribute, courtesy of Drew. It works via a bookmarklet making it really easy to use. Excellent work, that man.