This video of high school students in South Korea cheering their soccer teams is brilliant. Each of the 750 students wears a jacket that's a different color in front and back. Removing the jacket reveals a third color. Take that simple idea, throw in some well-timed choreography, and a giant TV screen is created, with each person acting as a pixel. The result is breathtaking.
If you were blown away by that, check out this video of the same idea, times 100, done in North Korea (jump to the 12:00 mark to see it).