The Gambit Fantasy Club is currently managed by BiggDoggProblem, and I believe it may be the most popular and best-managed club on the site.
The array of thematic gambit tournaments is excellent, and they tend to fill fast.
It is also good in the competitive sense. I have played over 300 games in club tournaments, and my club rating seems to stay 150-200 points below my regular RHP rating.
Either that, or I am an easy mark for Gambiteers!
@Paul-Leggett
I enjoy their slideshow of all the gambits, it took me a while to figure out what a few of them represented.
My favorite would be the Elephant due to the games of Phillip Corbin. (Calypso Chess)