The attendee badges at Defcon 15 can be programmed to display arbitrary words and phrases via their built-in LEDs using nothing but the on-board hardware — once you figure out how to hack them.
I just got my wife's badge for Defcon (they're not giving out press badges 'till tomorrow) and it is totally and completely awesome. At first when I put the batteries in, the LEDs lit up and then nothing happened. I tried shorting a few pins together on the back to no avail and then by accident I figured out that the front has two buttons, which are the smiley skull and the dial. After pressing the buttons it scrolled some text about Defcon, and then I hit the buttons a few more times and saw POV, which stands for persistence of vision. I swung the badge around and saw the word defcon, just like the cool spoke POV kits. Then I pushed the buttons a few more times and it displayed: TEXT. I held both buttons at once and I was able to program in 15 characters of text using the buttons to navigate and pressing them both at once to select a letter. I choose: "EECUE[HEART]PENELOPER^" with the heart being an actual heart symbol. The badge offers a full upper and lowercase alphabet along with an assortment of symbols and punctuation. After adding the 15 characters the text began to scroll in a marquee fashion. This is the dopest badge ever. Hell yeah and way to go Defcon!