Like most people, when I first started with Arduino I quickly went from making Pin 13 flash to more LED projects. I had always wanted to create an LED Matrix from when I first started playing with Micros 10 years ago. (I think once in school I might have done a 3x3 matrix on a breadboard.)
Originally I wanted to do an 8x8. Since I had a pile of 5mm Blue LEDs laying around, I decided to make an 8x6 since that is all I could fit on a PCB the size of a shield. I wanted a Shield because I didn't want to run wires everywhere and this was my first chance to try making an Arduino Shield.
Initially I started with a 5x5 on Protoboard. It was enough to try different coding techniques but the limited number of LEDs and low-brightness of the ones I used wasn't a very fun result.
After creating a PCB, using super-bright blue LEDs, and optimized code from all my learnings on the prototype, I ended up with this:
http://www.cmiyc.com/blog/2010/09/26/8x6-led-matrix-shield-for-ardiuno/
Shockingly, I am limiting the LEDs to 10mA and they are still retina burning!