One of Dan Zen’s most successful game was Blimp Races. In the first race, people created their own blimps and then e-mailed friends to join their blimps. The blimp with the most people is winning the race. The first race had about 700 people all having fun and cursing other blimps in the forums.
Six degrees of separation were explored as who people invited and where they came from were displayed. In the second race, two years later, it really exploded when invites went into silicon valley and reached out to India, Hawaii, the Netherlands, etc.
The third race two years later was more difficult as distrust in e-mail had grown to a maximum. People were being overwhelmed by spam and just not in the mood to click links in e-mail. This situation may have gotten better with spam filters and it might be worth a try to hold race four.
The race was almost sold and set to run on the large Bell/Sympatico portal but a change in management and a reduction of original content resulted in a no go.