Black Mesa (formerly Black Mesa: Source) is a Half-Life 2 total conversion remaking of 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 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.

The Black Mesa Project

Black Mesa was started by fans with the goal of remaking the original Half-Life game on the Source engine. The project began in 2004 after it became apparent that Half-Life: Source was not a complete remake but rather an enhanced port of the original game on the Source engine.
The team that survives today has been working on the project relentlessly for several years and are determined to see it through to completion.

More info: http://wiki.blackmesasource.com/Black_Mesa_Project

SYSTEM REQUIREMENTS

Minimum System Requirements

Software : Windows XP, Vista, or 7
Processor : Pentium 4 3.0 GHz, or AMD processor.
RAM : 1GB (1024 MB)
Graphics Card : Shader Model 2.0 capable, ATI 9600, NVidia 6600 or better.

Sound : DirectX 8.1 compatible.
HDD : 8 GB (Not including the Free Source SDK, installed separately)

Ideal System Requirements

Software : Windows XP, Vista, or 7
Processor : Intel Core 2 Duo 2.4GHz or AMD equivalent.
RAM : 1 GB (1024 MB)
Graphics Card : Shader Model 3.0 capable, ATI X1600, NVidia 7600 or better.

Sound : DirectX 9.0c compatible.
HDD : 8 GB (Not including the Free Source SDK, installed separately)