Megaman 8 Bit Deathmatch is a great mod for Doom 2, it can also run on Freedoom if you're like me and don't have 5 dollars lying around. (Can't work cause 14 :C)
Deathmatch Mode
In DM mode you, well, DM. You fight against other Robot Masters including Bass, Protoman, and Megaman.
User Created Levels/Modes
In these levels, it's either custom DM levels, or my favorite, Puzzle levels, which are essentially just the original levels from Megaman, but in 3D.
Download Here:
http://cutstuff.net/mm8bdm/?page_id=5/discuss
I know there's much more to the game but I'm in a hurry