In a post on WeBreakStuff entitled Wow, talk about error messages 2.0, there is an error message from Technorati that's pretty dang funny. Companies should use this kind of humor more often. It makes you care so much less about the actual error.
I think MySpace should take note. I see their error message more than any other site I visit (and perhaps more than all of the others combined!), and I think I'd be less annoyed if they just used some humor. I actually have no clue what their error message says anymore because I stopped reading it long ago.
And, it's not an error message, but I just love Gmail's "Hooray, no spam here!" message when your spam folder is empty. :)