I love this visualization you get in the (cross platform) Azureus bittorrent client. The central pie chart shows how much of the file that your downloading you have. Those smaller charts around the outside are other users, and you can see how data is being moved around by the animated blocks moving back and forth between the peers. A lovely simple visualization, useful in practice and also a great explanatory diagram on how the system works and why it’s so broadly useful, no need for all that confusing talk of seeds and leechers and so on.