Tribler.org:
it a cross-platform p2p client but it offer its open source API as well.
It implements many non standard features like P2P video streaming,
DHT, Merkle Hashes, repex, NAT and firewall puncturing, ...
Its engine is part of an European-founded video streaming project: p2p-next.
Unfortunately it's written in python, but you can wrap it in c++
or even generate c++ libraries out of python.
Alternatively: libswift
