Mega Man Maker (available for PC, Mac and Linux) is a fan project to do for Mega Man what Super Mario Maker does for Mario games, and it’s very well-made! It has a huge variety of levels available on its website to play, although what seems to be an account creation bug makes it difficult to tell you about them from experience. In addition to Mega Man (a.k.a. Rock), it allows Proto Man, Beat, or fan-favorite character Roll to be included in levels, each with their own special abilities.
It has a pretty cool tutorial as well, in which Dr. Wily and Dr. Light explain how the editor works. Their pixel artist really nailed that air of playful malevolence in Dr. Wily, who seems like he’d be fun to know if it weren’t for constantly trying to take over the world. Have a look:
I feel I should mention that it doesn’t contain all of the classic Mega Man series’ enemies and bosses, nor does it allow you to string levels together into a Mega Man adventure itself. As did Super Mario Maker, and SMM2 before its world creator update, it’s focused on designing single levels, although you can make them quite large, even big enough to be a whole adventure in itself.
Version 1.8 is due to come out in a few weeks, with a whole bunch of new features, and its creators have been busy hyping it up. We look forward to trying out the new version!