qBittorrent - Lightweight and open source torrent client that runs on all major platforms.
Baïkal - Cal and CardDAV server
Deluge - Deluge is a full-featured BitTorrent client for Linux, Unix and Windows.
Radicale - The Radicale Project is a complete CalDAV (calendar) and CardDAV (contact) server solution.
BitTorrent - BitTorrent is a peer-to-peer program developed by Bram Cohen and BitTorrent, Inc.
DAViCal - DAViCal is a server for calendar sharing.