http://users.telenet.be/d.rijmenants/en/enigmasim.htm
Best one I could find, and it simulates 3 and 4 rotor enigma machiens. Even better, it works with the real machines as well, so if you have a message and the machine presets, this sim will decode it.
For multiplayer this would be a problem, as you would need to swap back and forth between the game and the simulator to use it. Not impossible mind you, just difficult.