No More Room in Hell the long awaited Source zombie modification.
Very, very competent mod work on this title. Usually when a "mod" is great, I would call it just that. But the success behind this has to do with nothing else with a purely competent and professional dev team, with a big bunch of superb quality contributions all around. Still needs some work to reach toward perfection, but that is a matter of time.
9.5/10, would throw money at.