Lilac Chaser

According to Wikipedia:

When one stares at the cross for about 20 seconds or so, one sees three different things:

1. A gap running around the circle of lilac discs;
2. A green disc running around the circle of lilac discs in place of the gap;
3. The green disc running around on the grey background, with the lilac discs having disappeared in sequence.

Direct link; via Jeremy Hinton