We present ProtoPeer, a peer-to-peer systems prototyping toolkit that allows for switching between the event driven simulation and live network deployment without changing any of the application code. ProtoPeer exports a set of APIs for message passing, message queuing, timer operations as well as overlay routing and managing the overlay neighbors. Users can plug in their own custom implementations of most of the parts of ProtoPeer including custom network models for simulation and custom message passing over transports other than the default TCP/UDP.