You don't have to wait for Steam to publish Greenlight winner Black Mesa Source, the Half Life 2-based recreation of Half Life. The team has released the Half Life 2 total conversion for free through its own website.
So if the game's out and free, why does it need to be on Steam? The developers explain on the Greenlight page: "When we are on Steam we'll be able to reach more players and make installation and future updates easy for everyone!" Making updates more convenient is always a noble goal.