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.

Many useful plugins are available:

• Auto-adjustment of your upload and download speeds according to your network charge with Autospeed
• Auto-download torrents with RSS Feed Scanner
• Schedule upload and download speed limits with Speed Scheduler
• Receive a mail when a download finishes with Status Mailer
• Add numerous info on torrents (Extra seeding columns, Country Locator)
• Remote control Azureus through the Swing Web Interface or the HTML Web Interface