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

Main Features:
- 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.
- Many useful plugins are available.