Since the only thread for this game I know of is around 4 years old at this point and hasn't been posted in for about that much time, I thought that making a new thread for this would be good.
Old threadWebsiteTrailer for the latest version of the game.Mega Man 8-bit Deathmatch is a free project created by CutmanMike and a team of contributors consisting of hardcore fans of Capcom’s classic Mega Man series. Powered by the Skulltag engine, this retro styled first person shooter is designed around the look and feel of the 8-bit Mega Man games. It includes every robot master as a playable skin, over 50 weapons and maps based off the original games, 32 player online play, a singleplayer botmatch campaign and much much more. If you’re a fan of online first person shooters or Mega Man, you simply have no excuse to not download and give Mega Man 8-bit Deathmatch a try today!
That description is a bit off, since the game's switched to using Zandronum rather than Skulltag. While the game has a single player campaign, the multiplayer is where the game really shines. As of now, the core has skins, weapons and stages from Mega Man 1-8 as well as Mega Man and Bass and Wily Wars.
While you can access multiplayer servers through the internal browser, that's slower and the interface is clunky. So for that reason, I recomend using doomseeker, which finds and downloads WAD's you don't have for you.
Here's the guide to setting up doomseeker, writen by the creator of the game.I don't think there's anything else for me to say, though this OP could propably be better, since I'm not all that great at writing OP's.