They're waiting for you Gordon, at the Black Mesa Website, where they now offer the long-awaited first release of the Black Mesa modification to bring the gameplay from Half-Life to the Source engine (thanks ochentay4). Here's word from the wiki:

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.

This total conversion will not require Half-Life: Source to play - only a copy of any Source Engine game installed on Steam. For more details, check the System Requirements.