A few years ago I picked up an LED matrix panel from Adafruit, without really knowing what I was going to do with it. Well, with the amount of Minecraft I was playing at the time, anything grid-based was bound to get me thinking about how to hook it up with that game.
So I wrote a mod, using the then-popular Bukkit modding framework. This is probably the first (and as of this writing, only) thing I ever wrote in Java. It links a grid of blocks in-game to the LED panel, using Arduino to push the data. Just make a selection in game, fire a command, and watch your blocks appear in meatspace! I'll see if I can get this working again, and put some pictures up.
I later used this same LED panel in conjunction with my Hard Livin' art installation.
Languages: Java, Arduino
Future plans: Depends on whether Minecraft ever releases an official modding API, as Bukkit is no longer available. Aside from that I can only look for other weird sources of pixel data to display on the screen.