One project I have my eyes on are the 8x8x8 led cube with RGB color change as seen on instructables and youtube:
http://www.instructables.com/id/How-to-build-an-8x8x8-LED-cube-and-control-it-with/
http://www.youtube.com/watch?v=Aj3_v7xCyJ0
I've set up a 2x2 matrix and gotten some patterns going with different colors, as well as a 1x6 line of leds to flash through its range. For this project I am going to need some LED drivers, which are small chips preprogrammed to interact with the LEDs. With a few of those I will be able to regulate the colors and control many more leds than with the few I/O pins of the arduino.
I've got some other projects in the works as well but will save those for another blog.