I am going to go way back in time. I played so much Co-op Doom 2, both over my 28800 baud modem or a serial cable connection locally. We played through the main game and then downloaded a ton of "wads" (custom made levels) as I believed they were called. We'd have to negotiate with our parents so we could hog the phone line and play. One really had to work for their co-op action back then. I had so much fun playing that game.