Bitsharp is the collective name for a suite of bittorrent libraries which deal with every aspect of the bittorrent protocol. There exists libraries for creating/manipulating torrents, downloading torrents, manipulating the BEncoding format and hosting torrents to be downloaded by others.
They are all 100% cross platform and work on Windows, Mac OS X, and most (if not all) flavours nix.
These details are provided for information only. No information here is legal advice and should not be used as such.