Another interesting opensource project:
OverSim is an open-source overlay and peer-to-peer network simulation framework for the OMNeT++simulation environment. The simulator contains several models for structured (e.g. Chord, Kademlia, Pastry) and unstructured (e.g. GIA) P2P systems and overlay protocols.
OverSim was developed at the Institute of Telematics (research group Prof. Zitterbart), Karlsruhe Institute of Technology (KIT) within the scope of the ScaleNet project funded by the German Federal Ministry of Education and Research. The simulator is actively developed and open to contributions. If you have any questions regarding OverSim, please send an email to the MailingList.