One thing that really fascinates me are glimpses into the development process of older games and consoles (Specifically from the 8 to 64-bit era).
Personally, being able to understand (or at least look at) how game developers did things makes the idea of developing my own game for one of these consoles feel a lot more approachable... same kind of thing with the contents of a behind-the-scenes section you might find on a movie DVD.Here's a few interesting ones, starting with diagnostic cartridges
..the making of Solstice..
and a page on N64.com that includes early images of OoT.
[Page Link]If anyone would like to make their own thread for newer games, that'd be fine by me. This is more for looking at and learning from the past when video game consoles weren't quite as advanced.