I also love the Source Engine networking. Another one I like is The Torque Game Engine. It has a very similar networking model, I think it may even be a little

I also love the Source Engine networking. Another one I like is The Torque Game Engine. It has a very similar networking model, I think it may even be a little better (it's the engine that powered the original Tribes game). I remember buying a license to the original TGE years ago and digging through the networking code, it was great. They've open-sourced the latest version of Torque under the MIT license, and I believe the networking code is nearly identical to the original code used for Tribes.