I spent the last two days experimenting with the Source SDK tools to achieve what is commonly called a "Sin City" or "Noir" effect.
Being able to play a game with the stark black and white artstyle and storytelling the graphic novels are known for has been a dream for many years (yes, I know about Mad World).
The first result was a test of the $lightwarp shader function to give a cell-shaded look. As you can see in the picture below, the effeect ist suboptimal for brushes because of the stepped shadows source creates. Stencil shadows as seen in Doom 3 would be better here. The shader might work for models, though. Also, it is very hard to get the lighting to work right.
The following picture shows a test for spot lighting. Doesn't look good, does it? The main problem is that depending on where you set the threshold in the black-white sub-texture, a surface either receives too much or not enough light.
Lastly, a rather normal conversion to grey tones. I desatureated all textures in the scene except for the blood. This looks pretty good, but is in effect not that different from the normal game art.
That being said, I'll probably tinker with the setting a bit fruther and see if I can optimize the results.
Maybe there's someone out there who likes Sin City as much as I do and is interested in experimenting, or who might even start a full-blown mod project for HL2.