I think a bittorrent library would be a nice effort that is easy to communicate around:
* everyone is aware of Bittorrent * it's fairly low-level so no need to build tons of library around (say GUI) * it can be useful * lots of reference implementation The only issue is that you likely can just run c2nim on any of the C libraries and be done with it. Otherwise on the very useful and somewhat complex, an OpenCV wrapper or an Excel API wrapper.
