The Fate Codex will be a small electronic magazine featuring rules, hacks, fiction and settings from Fate creators across the board. It's no coincidence that Mark's talked to several of the folks who worked on the Fate Worlds books, since almost all of us have been hounding Fred about when the next ones should come out. I don't think Mark will have much trouble getting some awesome designers to put out some neat ideas for the game.
The hacks I post here are a good way for me to stretch my legs as a designer and not get wrapped up in my pro work all the time. Fate is a flexible and durable system and I'm very happy with the response I've gotten for my work here. I hope that similar work shows up in The Fate Codex, since working with Mark is a great pleasure.