From Ed Felten:
"TinyP2P is a functional peer-to-peer file sharing application, written in fifteen lines of code, in the Python programming language. I wrote TinyP2P to illustrate the difficulty of regulating peer-to-peer applications. Peer-to-peer apps can be very simple, and any moderately skilled programmer can write one, so attempts to ban their creation would be fruitless."
Find it here.