Mega Man 2.5D is a fan game developed by a small team of obsessive Mega Man fans. It began in 2009 as an animated proof-of-concept. The concept got a surprising amount of positive feedback, and as result of this we got a bunch of people together with the goal of making a playable game out of it.
There are mainly two things that set the game apart from most other Mega Man games. The first is the co-op mode, where two players team up to take on the robot masters. The other is the use of camera perspective changes which is featured a lot in the single-player mode of the game.
Feel free to check out some videos of Mega Man 2.5D here.