One of the biggest trends to emerge in game development over the past year has been a move toward involving players in the design process. One need only look at the majority of successfully crowdfunded games to see how quickly this is catching on.
Steam creator Valve is stepping up to embrace this movement with its Early Access program, which allows developers to sell their work-in-progress games to players to, in theory, receive feedback and ultimately ship a better game.
The sell for players is that they get to be the first to play new games, they get to interact directly with developers, and they get to see a game progress toward its final state. The sell for developers, obviously, is that you can foster and grow your community by giving them playable builds, and you can tweak your game early and often based on feedback.
Those with games already approved for Steam, either through Greenlight or traditional means, are encouraged to contact their Steam release team if they'd like to get in on the program. More information is available here.