Fans do some pretty incredible things given the right resources, whether it's developing their own sequels and spinoffs, to even completely remaking the original freaking Half-Life. As a matter of fact, that last one is exactly what the Black Mesa Source team have done, rebuilding the original game (up to the Lambda Core level, at least) from scratch using Valve's Source engine. From the project's wiki:
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.
A quick poke around the team's wiki and official forums proves the remake, which is dated for September 14th ("12 days! 12 days until the impossible!" as Alec Meer exclaims), is a definite feast for the eyes and seems to be living up to all the promise of the project.
Team frontman Carlos Montero added in his forum post announcement that future content is forthcoming, but for now, the team wishes to give fans "a complete 8-10 hour experience with a solid ending" while they work on Xen and BMDM. You'll be able to download Black Mesa from the project's website on the 14th, and until then, help yourself to a download of the pay-what-you-will original soundtrack.
Via Rock, Paper, Shotgun.