Station 51 is a single player mod for Half-Life 2: Episode 2. You are on the run from the Combine closing in on your outpost. Can you escape?
The environments are very well done, but with one major flaw: many of them don't seem to be/have been functional. As I progressed through the mod, I kept asking "Why is this room/space/object here? What purpose does it serve?" Still, very enjoyable and great-looking mod.