The last time I visited home, my little brother introduced me to a fun little Doom 2 mod called Megaman 8Bit Deathmatch. As the name implies, the game uses the Doom 2 engine in conjunction with Megaman characters and weapons to create a online multiplayer shooter.
Anyone who is familiar with either of the aforementioned franchises will definitely take some pleasure in this game. The gameplay doesn’t deviate very much from the original Doom formula, but tosses in tons of Megaman content to create an appropriately 8 bit environment to battle within. The greatest joy the game provides is the aboslutely ridiculous number of usuable weapons. So many classic Megaman standbys are available (literally dozens), including favourites such as the Gemini Laser and Bubble Lead. I spent a good hour playing through the levels, with the sole purpose of finding and utilizing every possible weapon.
Additional features include singleplay and offline modes, an inventory system that lets you use special utility items, and a bevy of maps made specifically for the game. Although it is a large download (about 50MB) it would be well worth your time to give MM8BDM a shot, at least for nostalgia’s sake. You can download it for free at cutstuff.net.