Koalas to the Max
koalastothemax.com
Move your mouse over circles and they split into smaller circles — keep going and an image slowly reveals itself, built entirely from thousands of tiny dots.
Why it is useful
A beautiful example of interaction design as art. The mechanic is simple and satisfying, the reveal is gradual enough to maintain curiosity, and the final image is a surprise. Demonstrates how a single well-designed interaction can produce an engaging experience with minimal complexity. One of those sites that is better experienced than described — share it without explaining what happens.