Black Mesa: Source, the fan-made source engine remake of the original Half-Life, has been in development for the better half of a decade now, and it finally saw an official release this past weekend. As I understand it, the last few levels are still yet to be completed, but what I've seen so far has really impressed me. The new visuals look just as good as (if not better than) the things Valve have been able to do with their own engine, with remarkable fidelity to the game's original designs but with all kinds of exciting new variations, as well.
It should be noted, however, that Black Mesa is not just a graphics overhaul -- it also spices up the gameplay by incorporating certain mechanics from Half-Life 2, such as a greater emphasis on physics manipulation and being able to light headcrab zombies on fire with flares. I've also spotted a number of retroactive continuity references to Half-Life 2 (such as a more prominent meeting with Eli Vance, among others), which help tie the two games together a little better and which serve as fun easter eggs for fans.
Perhaps most importantly, it seems that the development team made a deliberate effort to fix some of the illogical architecture and engineering that bothered me so much in the original game. At the very least, I've noticed the tram ride from the start of the game is now designed so that lab operations no longer obstruct the path of the tram, so I'm especially impressed with that. So far I've only played up to the "We've Got Hostiles" chapter, but playing Black Mesa has been a wonderful treat for my nostalgia, because it's almost like playing Half-Life for the first time again.
Anyway, I have a bunch of comparison screenshots of the original Half-Life and the new Black Mesa scenes for you to peruse in the full article, to get a feel for just how much work went into recreating the environs of the original game. Note in particular several occasions where there's now something where there used to be nothing, meaning there's a whole lot of new stuff to see and experience.
* Click each image for full-size