Donkey Kong II: Jumpman Returns

Jeff Kulczycki of ROM Hack created this game in 2006. It is a homebrew game having no affiliation with Nintendo and can be applied as a chip update to a Donkey Kong PCB. Adding the DK2 chipset allows you to switch from original Donkey Kong to DK2 via pressing both Player Start buttons followed by the Jump button.

The modded game offers a total of 8 levels. This includes the original 4 levels, with a few minor (clever) updates, and 4 new levels: Mixer, Foundry, Refinery, and Incinerator. I love the chip-swap implementation making this upgrade fairly easy for most owners. But what astounds me most is the visual integration of the 4 new levels!