Best Raspberry Pi build?

What is the coolest thing you've built with a Raspberry Pi?

I tried to make the Pi-Boy, but I couldn't make working buttons. I had to control it with a USB Keyboard.

Friend of mine just hooked up a mine craft server on a ras Pi, and then linked the Pi to a fire fly light, that gets brighter as more people connect to the server.

Thought it was a neat little nerd-DIY project.

Personally I just use raspi's for firewall systems.