I'm sorry if this has been covered before but it beats having to scourge the whole forum.  I'm new to this but I haven't seen any mods like this:

1.  Hot rod paint scheme (done before, i know) but with metal pipes going from the back like dual exhaust?  Not practical but cool though.  Great for those with dual processors.  You could also get parts from a Dub City or similar 1:18 car and get the blower from that, then add it to the top of the mac.  One could get a decible meter and put it inside, yet mount an outside gauge to the chassis.  The gauge could be water temperature or oil pressure.  One could go farther with cheap mods and add dice hanging from the front upper bar, dial-in numbers on the front like they put on cars at the dragstrip, even stickers from auto parts sponsors.  I'm sure some of you are rolling your eyes while others are going, "cool!" ha ha ha.

2.  Chase lights witin and around the motheboard.  i'm not referring to the wires the thickness as water coolant pipes, but fiber optics that could light up when there's disk activity.  It would look like veins pumping data throughout the computer.  If It could be slowed down some a bit to simulate the data going from point A to point B it would look even better.

3.  A face or logo etched on a Mac where the eyes of the face or significant parts of the logo would light up during dissk activity.  Even yet have an OR switch somewhere in the circuitry where one light would always stay on, but the other light or lights would switch on while the main light switches off druing disk activity.  I've got the "U" in unreal logo in my head but I'm sure you can think of other ones.

4.  I'm into building model cars (http://www.scaleautomag.com is a great place to go) and I've heard of places where you can send parts away to have chromed or rechromed.  Similar places do this for car parts as well.  It'd be cool to have either the grey or blue parts on my G4 set in chrome against the other color.  Might be a little pricey but definitely one-of-a-kind.  Silver paint doesn't do the same thing and paints that advertise themselves as chrome are really just a high-lustre silver.

5.  For those that have a B&W G3 or graphite G4, you can go for the lowrider look.  Get a belt sander and grind down the front lower support bar just a little bit (not sure if this will affect HD performance since the Mac will be at a slight angle), then get some pinstriping along the inside of the Mac case.

check out the worklogs at hardforum.com