Steam Greenlight is a new service that will let you, yes YOU, pick which indie-developed games make it to Steam in the future. From the services info page:
Valve is encouraging developers to publish their games to Greenlight as early as possible so that the game and the developers can gain as much support as possible before it's ready to ship. This will give the developers a better chance of seeing their games on Steam.
If the game is not accepted at any point, well, Valve is nice enough to let developers to continue to use their server space to advertise their games.
Valve's Anna Sweet commented on the difficulty of the publishing process sayingSteam Greenlight is set to be released on 30th August.