Not too many game Design Documents have mode their way onto the internet, but David Jaffe (he is a loud drunk) of God of War fame has put up the initial Concept Document of Calling All Cars in His Recent Blog Post. The game isn't a full A retail title, but it has been made into to a game that will be sold on Sony's PS3 download network (or whatever it's called). I was a little surprised to see how many times TBD (To Be Determined) had been written, but still very interesting.
For a little background: A Concept Document is a first stage document that literally just outlines the game to give someone an interest in the concept in ten pages or less.
Then is it is good enough (someone is willing to pay for the game to be made) then there is a real Design Document that outlines all the small stuff like the game's menus, levels, sound, AI and stuff. Then that is turned over to the Production Team who will make the game and over the course of the game's development stuff is added and the Design Document turns into a Production Document or Design Bible. So although people talk about two design documents, it's really three as the Design Document will turn from being under one hundred pages to over two hundred once the game is finished.