Before Tecmo and Koei, before Square met Enix, before Namco found Bandai - this chart seeks to catalogue the history of game studios - spinoffs, mergers, acquisitions, defections. It's also suitable for posterising on your laundry room wall.
The chart's creator, Eric Wall for Games Are Evil, advises that "this is by no means the be all, end all, definitive history of video game development studios. There is still room for additional studios and for clarification of individual studio history, as well as more connections between the shift of studio talent from one company to another." He's missed a few studios and publishers - notably Bethesda Softworks.
But as a family tree, or perhaps family amoeba, of how things got to this point, it's as good a start as any I've seen. If you're having trouble seeing it in full size above, here's the direct link.
Developers! Developers! Developers! - The Flow Chart [Games Are Evil]