Black Mesa is an unofficial remake of the original Half-Life game. It has been done by a group of 40 volunteers.
When Half-Life 2 was released in 2004, Valve re-released several previous titles using their new Source game engine, including Half-Life as Half-Life: Source. It featured the Havok physics engine and improved effects for water and lightning, but the level architecture, textures and models remained unchanged.
The team behind Black Mesa aims to completely recreate Half-Life making proper use of the Source engine. The first section has been released in 2012.