A file sharing network that uses virtual addresses (instead of IP addresses) and ant-inspired routing algorithms to protect the privacy of both downloaders and uploaders (in other words, defeat RIAA spy tactics). Version 0.3 has native builds for Linux, OS X, and Windows. Free software, open-source and licensed under the GPL.
Several people have described MUTE as the "third generation file sharing network" (Napster => Gnutella => MUTE, with each generation getting less centralized and more anonymous). There is also a MUTE available over at MUTE Wiki.