As of this writing, the wiki has nine "Tested" formats, four "Not fully tested" formats, and one "Untested" format.
Rather than get into the tested formats, we'd like to focus on the untested "Incarceration," because it is exceedingly ambitious. It basically takes thirty people and splits them into five six-person teams, then mixes in EDH, Emperor, Star, Planechase, and Archenemy. And some other things, too.
It's clear why this format hasn't been tested, but we think it's fascinating. It's not even clear how this would be run in the real world, but presumably it would involve a large room where each team could have its own table. And the Two-Headed Giant aspect would free up about half the players to roam around to see what's going on at the other tables.