Don't know about you guys but I love this kind of shit. Have wasted some time this week going through various classic games from my childhood and discovering some of the stuff that was removed or in some cases left in during development. It's interesting getting an insight into the technical side of how these things were made, and cool to see new aspects of games I thought I knew inside out.
I guess I'm not the first here to come across this site, but The Cutting Room Floor is dedicated to this stuff and seems to have more in-depth WIP content and such from old games than any others.
Donkey Kong Country - lots of unused levels, animations, and dialogue; also turns out there were significant 'patches' to later versions of the game, including altered levels to make the Japanese version easier
Banjo-Kazooie - those mysterious objects shown at the end of the game can be collected after all (image)
Wind Waker - so much cool shit, for example being able to fly off the top of the Tower of the Gods into a version of the sea you're not meant to be able to access complete with a weird early-development version of a Triangle Island; also many cut levels
Super Mario World - o gad
