W3C have come just upgraded to the stable version of their online validator service. With initial beta tests starting last August there has been some huge updates, including:

• Tweaked markup and style for indicating position of error.
• Tweaked markup and style for source display.
• The documentation has seen much improvement.
• The "fussy" parsing mode is no longer available and will be drastically improved before it comes back - if ever
• and loads more, check them out at http://validator.w3.org/whatsnew.html#t2004-05-07