The validators at
w3.org have been given a face lift. They are much prettier now than they were the last time I checked my pages. I use the
Markup Validation Service and the
CSS Validation Service very frequently. These tools are a great way to validate your HTML/XHTML markup and CSS.
Incidentally the css validator can be downloaded for use on your own system.
For info about downloading see ...
http://jigsaw.w3.org/css-validator/DOWNLOAD.html
I didn't see anything on a download for the markup validator but there are links to source code and lots of other documentation.