Nothing really cool in the social sense of the word but I have cut out my fan holes so that there is less obstruction to the airflow. I have been thinking about putting grills on them but I never put my fingers near them so it hasn't been a priority.
It did make it cooler though. And then I made a "tunnel" from my side fan to my ram so it just cools the ram. I made it out of cardboard, but it still works alright.
Is that what you meant by cool?

havent MADE any mods as such - being as most things i have needed are available already! got a new cooler master aquagate and a few UV lights and a light reactive chem to put in the coolant will be going on my new budget sempron 3100 system to replace the god-awful laptop i am having to use (celeron 2.5 GHz, performs like a pII)

I have seen those water cooler systems with the dye in it, makes a really cool effect.

Neons and LED bars are a good way to go, especially combined with any shiny bits and perspex see through areas! AKASA do cheap LED case fans which can give a nice glow to your PC

I've removed the side panels for easy access. I don't see the point in doing anything else because the machine is tucked away under my desk anyway where I don't see it.
Besides, I like the way normal machines look

dont forget you can do functional mods rather than asthetic mods. For example with a few cheapo fans you could prevent a PC from running too hot, and even make it safer to raise the speed a little...

What about the people that don't have their pc under a desk? Some people may think of modding as an art form.

The first PC i built I used a basic Antec case and put a clear window on it using a kit I bought at best buy, looked okay. My new comp I just bought a pre-modded case because it looks cleaner. But i've since added an LED bar and fans.

A clear window kit?? clear window to me involves screwdriver, griding bits, perspex and some industial strength adhesive.... A normal MOD is to grind fan holes in a standard case and put in extra cooling fans. LED fans can also be put in....