Mega Man 2.5D is a fangame developed by a small team of obsessive Mega Man fans. It began in 2009 as an animated proof-of-concept. This got a surprising amount of positive response, and because of this a couple of us got together with the goal of making a playable game out of it.
There are mainly two things that set Mega Man 2.5D apart from most other Mega Man games. The first is the co-op mode, where two players team up to take on Dr. Wily. The other is the use of camera perspective changes which is featured a lot in the single-player mode of the game.
You can check out some videos of Mega Man 2.5D here