Though DLC pre-orders are irrelevant, I'm one of those people who still consider them part of the game, and I don't like it being impossible to acquire them later.

For those who are interested, I asked Startrail to keep track of these pre-orders bonuses in his topic on free games.
I have pointed out all those I know, but if someone knows others, especially for games of few years ago, it would be of great help to have a list as complete as possible.