Matt offers a great post on the "magic" of Yahoo's Hack Days... I had the pleasure of attending the most recent Open Hack Day in London...
For me the most magical thing about it was my utter lack of involvement. ;-) I did not help organize, coordinate, oversee, judge, etc. I went strictly as a spectator... It's so cool when an idea takes on a life of its own... The team did an amazing job, perfectly capturing the spirit of the program (but with an appropriate and unique EU flavor...)
Reminds me: Tom Coates mentioned recently that he got a kick out of seeing the name Fire Eagle starting to do the rounds... It was the silliest name he could come up with. I like it.