A redditor discovered a new addition to Steam’s Mac library today: Half-Life. The classic FPS was first released in 1998, but never saw a Mac release, although Half-Life 2 (and every other Valve game since) has done.
I’ve verified that the game (hl_osx) will indeed install and run, and by all accounts seems to work well. The official store page still lists it as Windows only, so perhaps some official announcement will be made about this soon. It’s shaping up to be an excellent week for classic games going Mac.