Underhell is a Source Engine game developed by the Underhell Team in collaboration with We Create Stuff. Underhell Chapter 1 requires ONLY Source SDK Base 2007 installed. No Source game is required.
I am surprised at how good the game is and yet quite different from Half Life 2, gameplay wise. I thorougly enjoyed it and am looking forward to the next chapter. A lot of intense action and the story telling is very impressive as well.