aria2 is a lightweight multi-protocol & multi-source, cross platform download utility operated in command-line. It supports HTTP/HTTPS, FTP, SFTP, BitTorrent and Metalink.
GPL-2.0 License
sophisticated command line file transfer program (ftp, http, sftp, fish, torrent)
stable, high-performance and low resource consumption BitTorrent client
A cross-platform FTP/FTPS client library based on Boost.Asio
an efficient feature complete C++ bittorrent implementation