RealtimeGameLib is a simple library for building multiplayer realtime games using P2P (and Flash Media Server). Some of its features:

• P2P Game class: handles movement, position, mouse position, rotation and speed callbacks back to your game. Allows you to distribute realtime data to everyone in the group using RealtimeChannel class.
• FMS Game class: creates a Flash Media Server game. It requires an application installed on FMS server.