An advanced, decentralized, java-based client for torrent files that is published under GNU/GPL license. It has a plugin archetecture and, while a little more advanced than some clients, it's easy enough for most average users to operate. It is regularly in the top 5 most active projects on SourceForge.
Negatively, the program can be bulky for computers with low amounts of RAM and often takes longer to load than other thinner clients such as the official software and uTorrent, but the customizability, long list of features, and available plugins often makes up for this.
Azureus was recently modified to create the Localhost Program, which creates a world-wide file system with a web-interface.
Azureus was started at Sourceforge mostly to experiment with the Standard Widget Toolkit from Eclipse, in June 2003. The program is multi-source, multi-platform and features multiple downloads. Azureus Japan Is the home page for the Japanese localized version.