MonoTorrent is a cross platform and open source implementation of the BitTorrent protocol. It supports many advanced features such as Encryption, DHT, Peer Exchange, Web Seeding and Magnet Links.
Notable features include:
-Peer Exchange (uTorrent style)
-Local Peer Discovery
libtorrent is a feature complete C++ bittorrent implementation focusing on efficiency and scalability. It runs on embedded devices as well as desktops. It boasts a well documented library interface that is easy to use. It comes with a simple bittorrent client demonstrating the use of the library.