Released: 1994, Super Nintendo
Donkey Kong Country is an SNES platformer released to much hype. It has the distinction of being the highest-selling non-bundled Super Nintendo game ever. Its graphics were created by capturing 3D models rendered with Silicon Graphics equipment. Those graphics were then packed into the ROM with a proprietary compression technique. This process came to be known as "Advanced Computer Modeling", or, "Magic".
As loaded as the game is with content, there's still more in the cartridge that remains unused. This includes unused bonus rooms, enemies, items, and scenery. Come read unused dialogue from that cantankerous chimp, Cranky Kong! Marvel at the unused animations from various characters! Like fonts? This game is loaded with them! A free movement debug mode can be enabled as well.
All this and much, much more in the Donkey Kong Country article.