If you're into games the way we're into games, you're less a fan and more an obsessive. That means you probably care a lot not just about the games themselves but also the people who made them.
Alas, the tree of game development history is gnarled indeed. It'd take some kind of savant to keep all the relationships straight, the who-bought-whoms and the which-group-went-on-to-found-what-indie-studio and such. Have you ever wondered what happened to all those internal Sega studios with the awesome names like Smilebit, Hitmaker, and Overworks? Ever tried to get a handle on the entire "2K [Place Name]" cabal? Are you shocked to find out that Killzone developer Guerrilla used to be called Lost Boys, or that those fine lads at Rare once labored under the moniker of Ultimate: Play the Game?
Then boy howdy, is this chart for you! It's an absurdly detailed diagram showing more game development companies than you can possibly shake a stick at. Note the legend at the bottom, so you can distinguish "sold" from "name change" and "formed by former members of."
Anyway, this amazing chart is courtesy of what must have been weeks of toil and drudgery by Games Are Evil, which says that this thing is by no means "definitive." But it's a damn good start. Give 'em a few clicks for their effort; you can hit their original story to see the full-size version of the image, and leave a comment if you have any additions or clarifications to make (the lack of Bethesda is kind of jarring, I have to say).
Or you can just marvel at exactly how damn many companies it takes to make this whole crazy industry go. I'd love to see a searchable version of this, since I still haven't managed to figure out if Psygnosis is listed on there anywhere or not. (EDIT: Yep, right in front of my face. Thanks to everyone who pointed out I should eyeball the screenshots I take before I comment on them.)