Because these projects are managed with revision control software such as CVS, Subversion, or increasingly, Git, the entire history of who added what, when, is recorded and available. Michael created a tool he calls code_swarm to take that data and visualize it, to beautiful effect. Below is the resulting video for the Python programming language (direct link).
This note from Michael will help you understand what’s going on a little more quickly:
This visualization, called code_swarm, shows the history of commits in a software project. A commit happens when a developer makes changes to the code or documents and transfers them into the central project repository. Both developers and files are represented as moving elements. When a developer commits a file, it lights up and flies towards that developer. Files are colored according to their purpose, such as whether they are source code or a document. If files or developers have not been active for a while, they will fade away. A histogram at the bottom keeps a reminder of what has come before.