Black Mesa: Source is a Half Life: Source modification, which is still in development. About the game, taken from Black Mesa Source official website:

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.

Official Trailer:

The first thing that makes us want to play the game, are, of course, the detailed graphics of the game. Take a look at these screenshots, and compare them to the original HL graphics.

Another interesting thing about the game, is that it will have 3 built-in different game modes.

Single-player

• Assuming the role of Dr. Gordon Freeman you will experience an immersive, story-driven mode of play. A world where around every corner a new threat or situation presents itself, provoking thought and interaction between you and your world. Relying on more than just combat to survive, your wits and intelligence will become strong allies in your fight for survival. Environments will be interactive, rich with detail, fully ambient and NPC characters will react to these environments and your actions therein. A world driven by story, locations conquered by wit, enemies matched with firepower. Welcome to Black Mesa. Run, think, shoot, live, Source.

Co-operative

• Black Mesa co-op will allow players to not only recruit the assistance of security guards and scientists, but also that of your friends! Share the single-player experience by battling through Black Mesa together. Players will be able to wage battles, travel corridors and emerge victorious over the enemies and escape the confines of the Black Mesa Research Facility.

Deathmatch

• The adrenaline pumping, bullet dodging, rockets-flying, big-red-button riddled deathmatch is back in classic Half-Life style, featuring graphically enhanced multiplayer maps and the full lineup of weapons. You’ll find yourself having more fun blowing away your friends than, well, anything.

Links:

Official site

Steam group

Youtube channel