If your talking about Doritos crash course both 1 and 2 were free, I don't know if they still are. So all you have to do is search for the game and they still might be free.
I would like to see Dead Rising 2 or 1 or Far Cry 3 or even Dead Island doesn't matter. They need to put some type of zombie game out there to appeal to the whole zombie crazy right now.
I doubt State of Decay will be a free release, their going through a lot of different changes/upgrades and formats to that game.
As far as halo 3 you know Microsoft they may wait till the last minute, but anything can happen...