One Validator for Everything
Well . . . not quite everything . . . for most things.
The World Wide Web Consortium has created a single-stop validator called Unicorn. From this single page, it allows you to check HTML, CSS, MobileOK, or your feeds — or your can check them all at one time. Developers are also encourage to develop their own modules, which could be added to this service.
I am constantly moving back and forth between validators at the W3C; having a single URL to remember is wonderful!