I just stumbled upon this nice experiment by Matthieu Latapy. If you have a website of your own (or better a blog), you can participate by clicking “spread it” and copying the code from this flash animation.
What I like about this experiment is that as a participant you can actually see how your participation influences the dataset (the graph should change as the code spreads) by exploring the graph in the animation. This is fun! – something that should not be underestimated in the design of experiments that depend on voluntary action. At the same time, your participation produces useful data for research. The originating site is http://www.happyflu.com. If you participate as well (and you should!), do not put any trackbacks to this or the happyflu site.