http://www.cs.wisc.edu/graphics/Gallery/NPRQuake/
NPRQuake is an NPR (non-photorealistic) version of Quake for Windows (and also later ported to *NIX), improved by Alex Mohr, Erik Bakke, Andrew Gardner, Christopher Herrman and Steve Outcher. Apparently some people have managed to get it work on other platforms, too, but the development by the original team seems to have stopped.
NPRQuake supports Sketch, Blueprint and Brushstroke rendering modes; This makes Quake look like a quick sketch, a blueprint or ballpoint-pen drawing. New rendering modules can also be written, added and used with not much difficulty.
This mod goes definitely in the category "Does nothing important in particular, but it's definitely a neat hack." Non-photorealistic rendering is not used much in the games, mostly because it's challenging to make it look good, more so than "realistic" shading. NPRQuake definitely has high hack value.