Azureus

Azureus implements the BitTorrent protocol using java language and comes bundled with many invaluable features for both beginners and advanced users:

* Multiple torrent downloads
* Upload and download speed limiting, both globally and per torrent
* Advanced seeding rules
* Adjustable disk cache
* Only uses one port for all the torrents
* UPnP sets the forward on your router
* Can use a proxy, for both tracker and peer communications
* Fast resume
* Can set a default download dir and move completed files
* Can import torrents automatically from a set dir
* Highly customizable interface
* IRC plugin included for quick help
* Embedded tracker, host your own torrents, automate your shares (periodic scanning of a dir)...
* Distributed Hash Table: host on it a completely decentralised torrent and/or benefit from it when the tracker is offline.

Linux
Windows
Mac OSX
Filezilla

FileZilla is a fast and reliable FTP client and server with lots of useful features and an intuitive interface.

Linux
Windows
Mac OSX
Frostwire

FrostWire, a Java Gnutella Peer-to-Peer client, is a collaborative effort from many Open Source and freelance developers located from all around the world. In late 2005, a few concerned developers of LimeWire's open source community announced the start of a new project fork "FrostWire" that would protect the developmental source code of the LimeWire client and any improvements to the Gnutella protocol design. The developers of FrostWire give high regard and respect to the GNU General Public License and consider it to be the ideal foundation of a creative and free enterprise market.

Linux
Windows
Mac OSX