http://www.virtualhideout.net/cool_case/ has hundreds of case-mods by users. The creativity and engineering that some of these projects entail is pretty fantastic. I have a custom case mod project of my own planned. Since I'm finally close to putting an MP3 box in my car (P-166 w/64 mb of RAM and stripped down 2.2.14 kernel), I've decided that I might as well make it look good.

It's a desktop box, so I'm planning on putting in three perspex windows, one on the top, and one on either side of the top casing. Then, I'm going to install a colored neon lamp (probably blue) inside.ThinkGeek (cool folks) sell 10" and 15" neon lamps that have Molex connectors, and take 12v DC. Since the trunk tends to get pretty hot in the sun, I'm also thinking about adding dual fans in the back to help keep things cool. I'll post pictures here soon.