A while ago I finished a simple multiplayer test demo using XNA 3.1 and Lidgren Network library. It works well and the gameplay is simple. One hosts, others join. Up to 8 players can play against each other.
Here's a little video of the test.
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Now I will experiment using Box2D and Lidgren Network together :)
AgentGrautvornix
I really admire your programming skills... :)
I hope you're gonna make a game out of this and will release it soon! :D