This is the case mod I've done to my desktop computer, Yoshi. The computer in question is an Athlon XP 2000+ in a full tower case. I installed a rectangular translucent window in the top, and a pac-man-shaped window in the left side. Why pac-man shaped? Good question.
There are 64 LEDs under the top window, and 8 LEDs under the side window. All were high brightness. (around 2000 mcd) On the top there are 40 red LEDs arranged in a 5x8 grid, 8 yellow and 8 green LEDs each in arcs on the top and bottom, and 4 each of blue and white LEDs in the corners. The side panel has one white LED under the 'dot' by the pac-man, and 7 other red, green, and blue LEDs under the pac-man. There's also a 16x2 character LCD on the front of the top panel.
It's all controlled by one PIC16F87 microcontroller recieving data from the PC's serial port.
I'll put up more information and schematics when I have time, for now you'll have to rely on the description in the firmware's source code.
The tarball up for download now contains firmware source and binaries, some simple test apps, and a very simple XMMS plugin. All the PC software is just a quick hack and the source code is very messy. I'll write a daemon and library for it when I have time.