Black Mesa is a mod built upon Valves Half life engine, it is a complete re-work of the Half life source. It achieves a level of detail that surpasses Half Life, In terms of level detail and map sizes. It should be noted that this is not a simple port of half life but is in-fact an entire rebuild from the ground up.
The goal is to remake the storyline used in Half life and turn it into a new polished Source Engine version with new models, maps, soundtracks, voice acting and textures.have a look at the graphical differences from the reworked engine below.
This game only requires a copy of any Source Engine game installed on Steam from this list.
The game itself is available from here but the site is suffering from a high volume of traffic, so you might have to wait awhile to get your copy.