Mega Man 8-bit Deathmatch is a fan-made Doom2 mod that basically turns the world of Mega Man into a FPS. You get to play as Mega Man or the robot masters and kill each other with special weapons from the classic series. Looks pretty good. I'm downloading the demo right now to try it out :)
Here's the trailer for the weapons.
You can get the demo here http://cutstuff.net/blog/?p=2218