I only got my Arduino a while ago but don't have many components at the moment to mess about with. Last week I got an 8x8 matrix through the post however, and have been playing with that.
After making some scrolling text, a php script to convert 8x8 images to hex arrays and such I decided I could make a fairly simple implementation of pong using the matrix (http://www.futurlec.com/LED/LEDMS88R.shtml), MAX7221 driver, a potentiometer and my Arduino Duemilanove.
As for the game, it's fairly basic pong. You vs AI (the AI isn't 100% accurate meaning you can win, but not often). The ball speed increases exponentially up to a certain limit to keep round times down. When someone scores 3 rounds the game is over, letting you know the final score and all the game variables are reset.
Here's a little video of it in action:
As you can see, it's not the best; The bat hit detection could be improved, the ball movement could be tweaked and currently the AI is almost impossible to beat.
I definitely need a bigger breadboard too!
I took the scrolling text off it in that example, but it does have a lovely nuke animation when you lose a point which was generated by my php script and friends paint skills.
I'll soon be near a maplins, so I might be able to grab some more components and make something a little more on the useful side in future!