These 3d animations of Linux kernel development [via Slashdot thread] show the growth of sourcecode and dependencies like nothing else. The guided tour of Linux-2.4.5 gives a great intuitive picture of the comparitive complexities of different portions of the code -- Lion's Commentary on Unix is a good complement to this (reading the surprisingly tiny piece of code responsible for a file node or the boot sequence gave me an emotional response similar to learning about Schrodinger's wave equation).

Better is the animation of the growth of Linux from 1.2.0 to 2.4.1. You can really see where the complexity comes in, which portions of code are similar to which others. Incredible. And even if all of this makes no sense, the animations are still beautiful to watch -- to have this kind of data available to model for other systems!