Microsoft announced Xbox Game Preview during their E3 press conference, a program which lets players buy and play game as they are in development, not unlike the popular Steam Early Access feature on PC.
So far, the games that are available for this program are, The long Dark, Sheltered, Elite Dangerous and Day Z, though more titles will almost certainly appear as time goes on. This should give indie and AAA developers alike a chance to fine-tune their games while still receiving funding from players. Microsoft noted that an extensive filtering system would be put in place to weed out poor titles and free demos would be mandatory.
Microsoft also said that players can begin joining this afternoon, marking the latest instant release announcement from the Xbox team.