Utilizing the Source engine, Black Mesa will reintroduce the player as Doctor Gordon Freeman, along with the original cast of memorable characters and environments seen in Half-Life. Black Mesa was built and founded on the basis that Half-Life: Source didn't fully live up to the potential of a Source engine port of Half-Life. As such, Black Mesa was founded to fully reconstruct the Half-Life universe utilizing Source to its fullest potential in terms of art detail, level sizes and code features. It should be noted that the project is being built from the ground up and is not a simple port of Half Life maps and models into the Source Engine. The idea is to remake the storyline used in Half Life into a new polished Source Engine version with new models, maps, soundtrack, voice acting and textures.
To play Black Mesa all you'll need is the 2007 Source SDK which you can install by going to Library > Tools menu in Steam.