I know it's not actually a Valve-produced game, but the Half-Life retelling, Black Mesa, certainly has all the trappings of one. From an immersive story and solid action to impressive dialogue and great character animation -- just listen to the scientists discuss the disaster that was the previous Black Mesa experiment; such amazing attention to detail.
Black Mesa isn't a carbon copy of Half-Life, it's more like a retelling, or reboot, of the original story. Your Dark Knight Rises to your Batman and Robin, if you like. If you haven't downloaded it yet, get on over there and grab it. It's free, and well worth playing, and not just for the nostalgia trip, either. [Kotaku]