I've just found a cool little Java app (installers available for Windows, OSX) for validating web sites called the Total Validator Tool. And it's free (though not open-source) so it's even better. Another tool to add to the utility belt.

As a typical sign that a major software product is ready for launch, mozilla.com is down and the release of Firefox 3 had been rumored for Any Day Now, so I guess half of the known universe tried to check all at once and took out their servers.