http://www.downforeveryoneorjustme.com/

Edit - I just tried it with Geocities.com, which no longer exists. Yahoo announced they would be discontinuing the site, and navigating there currently results in a "not found" error.

However, http://www.downforeveryoneorjustme.com indicated that it was "Just Me".

I don't know if it works for other sites, but in this particular case it didn't work.