It was single player (I think), futuristic, and the early version released took place in a city. that sounds kinda vague, but honestly it's the only single player mod I can even remember from Quake 3.
EDIT: To be a little more specific, when I say futuristic, I mean in a similar style to Perfect Dark. I remember there was at least a small download released. I can't remember if it had enemies in it, but you had at least 1 unique gun, and did have a smallish city environment to move around in, which sort of resembled the steel blue futuristic Perfect Dark city environments.
Also, I seem to remember it being made by a company known for something else. Like maybe if Wild Tangent made a mod, or some sound card company or something. I recall the website being sort of slick and professional looking.