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

eMule is a peer-to-peer file sharing application for Microsoft Windows. Started in May 2002 as an alternative to eDonkey2000, eMule now connects to both the eDonkey network and the Kad network. The distinguishing features of eMule are the direct exchange of sources between client nodes, fast recovery of corrupted downloads, and the use of a credit system to reward frequent uploaders. Furthermore, eMule transmits data in zlib-compressed form to save bandwidth.