Warren Robinett when he was making Atari Adventure wanted to make it 2 player. He had this crazy idea of connecting two 2600's using a cable that plugs into the 2P joystick port, but it never came to be. So, I have revived that dream - not as an Atari hack, though. Atari Adventure's been ported to C++ and runs on Macs and PCs, but it's now an internet-based, head-to-head version.
See the cool (and hilarious) promo video here.
It should be ready by Fall, but if people want to try it earlier I am looking for beta-testers. Find out more at http://h2hadventure.ddns.net.