From the Black Mesa Devs
"We have gotten a lot of remarks, questions and demands that we get Black Mesa on Steam. In the past, the answer has always been the same..."It's not our decision!" Well, it still isn't our decision. But now it's yours.
Black Mesa is now on the Steam Greenlight service. YOU can now be a part of helping get the game on Steam. Not only through your votes but through your help in getting all your friends and peers to vote for Black Mesa!
The more votes we get, the closer we will be to not only getting the game on Steam, but getting Steam integration. This doesn't just mean easier installation and updates for everyone, but it means a host of other advantages such as Achievement Tracking, Stat tracking, Steam Cloud, (eventual) Matchmaking services, Steam Community features, and a host of other useful things.
In short, it will help us make Black Mesa more awesome for you guys! So get out there and help us do this!"