This one is really interesting. Stamen Design took one day of digg.com and visualized the user activity. The explanation is really cool, and can be found here on content.stamen.com. Stamen works with Digg on a number of the tools available in Digg Labs.
Red dots represent people that have been on Digg for a while, yellow is a good mix, and blue is new users. So when a story is predominantly blue, it represents a story that is attracting a bunch of new users to Digg.