Black Mesa (formerly Black Mesa: Source) is a fan-made recreation of the original 1998 game Half-Life, made using Half-Life 2's engine. If you never played anything in the Half-Life series, you should have. First-person science fiction shooters, they follow MIT physicist and incredibly good-looking guy Gordon Freeman through alien-destroying adventures. In the first, he explores the Black Mesa science facility as it is being torn apart by aliens and the military in hopes of stopping an extradimensional invasion. The sequel introduced one of the best physics engines ever seen at that point, and would have been a fascinating and terrifying shooter even if it hadn't done that.
These are the kinds of games that inspire so much love that fans would try to meticulously reconstruct the original using the tools provided by the second. It's a little like if someone tried to recreate Morrowind using the Skyrim engine, or to recreate Super Metroid using Metroid Prime 3's engine. (If anyone's doing either of these, by the way, let me know. I want in.)
The following side-by-side might give you an indication of what kind of improvements they've made graphically:
|As seen in Half-Life . . .|
|. . . and fourteen years later in Black Mesa.|
|Both games are about fifteen seconds of anything related to a PhD from MIT and the rest is mainly blowing things up.|
If you've been waiting for Black Mesa, it's what they said it should be, and you should go play it. If you didn't know about it, go play it. You'll need the Source engine to play it (details below), and it's otherwise free. If the Half-Life universe is totally new to you, check it out. It's a staple of video game history, and this should be a very visible marker of what a fan community can do with a well-loved and well-crafted story when they get the chance.
Black Mesa: http://release.blackmesasource.com/
How to get Source (requires Steam): http://www.youtube.com/watch?v=ijuERFX-tY4