libtorrent is an open source implementation of the BitTorrent protocol. It is written in and has its main library interface in C++. Its most notable features are support for Mainline DHT, IPv6, HTTP seeds and µTorrent's peer exchange.
libtorrent uses Boost, specifically Boost.Asio to gain its platform independence. It is known to build on at least Windows, Mac OS X, Linux, and FreeBSD. In many package managers this library is known as libtorrent-rasterbar or the older rb-libtorrent.
Its original author is Arvid Norberg.