Black Mesa got on steam as a standalone title.

http://store.steampowered.com/app/362890

Basically what this is, is what started out as a love letter to the original half life by a group of modders who decided Half-Life source sucked and they wanted to properly rebuild Half-Life to look more like Half life 2, and I think in... 2012 it was released as a free mod in an incomplete state. Let me tell you I was blown away when I played it, so to see that it was picked up on steam as it's own game makes me really excited. I'm downloading it right now because I got paid tonight and instead of GTA 5 I decided on the DBXV Season pass and decided that I couldn't miss this lol.
Half life 1 was the first FPS game I ever played and it's always been a favorite game of mine.