A mod for the original Half-Life, Mistake -1, was released just the other day. Here is Planet Half-Life's take on the mod that has long been in development.
There is a list of download mirrors at the ModDB download page for the modification.
This is where Mistake -1 comes in. Now, it's just a ModDB profile page and has no website, which I instantly counted against it, but after actually playing it, I have to say I'm quite impressed. If you played through Afraid of Monsters and thought it could've been better, this is the mod for you.
There is a list of download mirrors at the ModDB download page for the modification.