A couple of years ago Mike Dailly released the design document for the original Grand Theft Auto video game (then called Race’n’Chase) on flickr.
It was planned that Race’n’Chase would be a multiplayer racing game first, but later evolved into what it is now.
On IGN there is a page dedicated to the canceled original concept for the game Monster Knight from Insomniac. The design bible can be found on the second page.
On the website of Jordan Mechner two samples of his journals can be found about the creation of Prince of Persia and Karateka. The complete journals can be bought from his website, but these samples already give a glimpse into developing these games.
At the bottom of this page the source code for the original, Apple II Prince of Persia and an explanatory technical document can be found.
The Doom Bible is the original design document for Doom, written by Tom Hall in 1992. Not much from this document was used in the original game, but it’s an interesting read.
Aric Wilmunder has released many of the original LucasArts Game Design Documents on his personal website. He is still busy adding new documents, but the following files can be seen on his website at this moment:
EDIT: Aric Wilmunder’s website seems to be offline, luckily the Internet Archive made a snapshot of the page with the game design documents
For the 20-year anniversary of WipEout (PS1) the game designer Nick Burcombe, who worked at UK developer Psygnosis, has released the official Game Design Document.
Deep Silver Volition has uploaded the Game Design Document and walkthrough for the unreleased Saints Row Undercover PSP game.
The developer for Monaco has released the original game design document on Facebook.