We just announced the addition of an Early Access section to the Steam store, which includes the availability of five previously Greenlit titles: Gear Up, Gnomoria, Kenshi, Kinetic Void, and StarForge.
Steam Early Access allow the community to get involved early and play select titles during their development. The goal of Early Access is to provide gamers with the chance to "go behind the scenes" and experience the development cycle firsthand and, more importantly, have a chance to interact with the developers by providing them feedback while the title is still being created.
To support the interaction between Early Access players and developers, Steam offers easy and automatic updating of games, letting developers iterate quickly to respond directly to bug reports and feedback from customers. And, like all Steam games, Early Access players will be able to interact with other players, making it easy to create and share screenshots, tips, and in-depth guides.
We're starting out with a dozen games and will be expanding it over time. If your game has already been Greenlit, then get in touch with your Valve release team and let us know of your plans and interest.