From Black Mesa: Wiki
Overview
Black Mesa (formerly Black Mesa: Source) is a Half-Life 2 total conversion remaking Valve Software's award-winning PC game, Half-Life. 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 was inadequate as the Source engine port of Half-Life, regardless of the intentions of Valve Software. 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. This total conversion will not require Half-Life: Source to play - only a copy of any Source Engine game installed on Steam. 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, voice acting and textures.
Game