(http://upload.wikimedia.org/wikipedia/en/4/44/Black-mesa-logo.png)
http://www.blackmesasource.com/ (http://www.blackmesasource.com/)
It's been 8 years since when it was first announced, and it's finally being released on September 14th, 2012. In addition to that, through Steam's Greenlight project, it has been overwhelmingly voted to have Valve's official support.
http://steamcommunity.com/sharedfiles/filedetails/?id=94128543 (http://steamcommunity.com/sharedfiles/filedetails/?id=94128543)
Kudos to the development team who spent years working on this on their own time, and the best part? It's completely free.