Tribler: social P2P system helps you share files and bandwidth with friends

I love this stuff -- I once co-founded a software company called OpenCola that was really, really similar. They're looking at all the hard technical and social problems of P2P, including finding and discarding fake or corrupted files. The LGPL license means that anyone can implement, improve upon and distribute the technology.

We have developed a new cooperative downloading protocol which makes use of social groups, where members who trust each other cooperate to improve their download performance. Peers from a social group that decide to participate in a cooperative download take one of two roles: they are either collectors or helpers (see Figure 1). A collector is the peer that is interested in obtaining a complete copy of a particular file, and a helper is a peer that is recruited by a collector to assist in downloading that file. Both collector and helpers start downloading the file using the classical Bittorrent tit-for-tat and cooperative download extensions. Before downloading, a helper asks the collector what chunk it should download. After downloading a file chunk, the helper sends the chunk to the collector without requesting anything in return. In addition to receiving file chunks from its helpers, the collector also optimizes its download performance by dynamically selecting the best available data source from the set of helpers and other peers in the Bittorrent network. Helpers give priority to collector requests and are therefore preferred as data sources.