An all-in-one validator from the W3C: markup, CSS and feed validation.
The W3C validator—one of the most valuable tools in the web developer toolkit—is in danger. Please help out. And please spread the word.
Henri Sivonen's HTML5 validator has now been integrated into the W3C validator.
For those times when you need to validate your markup but you don't have a 'net connection.
The Validator got a new lick of CSS paint and it's looking good.
The W3C Validator now has an API. It's SOAP only unfortunately, but this could still prove to be immensely useful for rolling into a CMS.