For you Mozilla users out there: The current Project of the Week over at mozdev.org is Checky, a great little add-on that makes validating your pages so easy you’ll have no excuse not to. Checky adds a submenu to Mozilla’s context menu that allows you to run whatever page you’re on through one of (currently) 18 different online validation and analysis services.
Even more useful, it includes Checky-Agent that will run the current page against any one or more of the services you select, all at once. Checky-Agent can be activated from the Checky menu or by simply pressing the F10 key. So, for example, you could run your page against the W3C’s markup and CSS validators, the Web Design Group’s HTML Validator, and Watchfire’s Bobby with a single keystroke.