Rockman: Deux Ex Machina is a pretty extensive ROM hack of Mega Man 2, both graphics and gameplay-wise. While most of the bosses and enemies remain the same, the power-ups and level designs have significantly changed, with gameplay mechanics added or modified to make the game freshly challenging such as flickering rooms or more hostile scenery. It also includes music adapted from other Mega Man games and even games like Metal Slug and Super Puzzle Fighter. You can read this list (Japanese only) for a list of all the songs used in the romhack.
Here, woabclf zips and hacks it through all the levels and bosses as fast as possible and finds themselves encountering a surprise enemy.