If you have an able-bodied computer, own Half-Life 2: Episode 1 (which, if you have a computer that can run it and you're on this website, why the fuck don't you?), and like the single-player aspect of Half-Life, you should probably play Minerva. Probably one of the best single-player FPS experiences (besides Half-Life 2 and episodes, Bioshock, and a handful of others) in recent memory, Minerva has some of the best level design I've seen in a while, with amazing puzzles and a great, mysterious storyline that runs parallel to Half-Life's.
Anyway, enough about the mod, you can read the Valve Developer community article if you want to know more. The real reason of this post was to say that the creator, Adam Foster, recently posted on his blog that he will be working for Valve starting early October. This is exciting one, because we now know that Episode 3 isn't that far into development if they're still hiring people to work on it (BAWWW), and two, because Mr. Foster is a genius.
My only qualm with this is hopefully, Adam will finish off the Storyline he had going with Minerva. I'd be pretty upset if he just left the highly ambitious next chapter for vaporware.