I missed the original Half-Life when it came out way back in 1998, but I did play through it some years later. Black Mesa is a total conversion of Half-Life 2 back into the original—basically a remake of the original Half-Life with updated graphics and art.
It's good. Very well done. It's loyal to the original material, though not afraid to streamline here and there. The Source engine is now, what, eight years old? but it still looks amazingly good. I remember the original game dragged a bit in the middle, with so many endless tunnels and catwalks, and this one feels like it plays a bit more smoothly. The custom soundtrack is a bit loud sometimes, but sounds very professional and mostly appropriate. There is a small disappointment in that the final level is not available yet, but they say it's coming soon. Overall, it's extremely impressive.
Most interestingly, this is a totally volunteer production that is available for free (see above link), so long as you have some Source engine-based game (such as Half-Life 2) already installed. (And there are of course pirated stand-alone editions without this requirement.) I'm always pleased to see when these kinds of multi-year labors of love turn out this well.