BitLet is a bittorrent client which runs in the browser as a Java applet, thus requiring no local installation. I built it with a friend (hello Daniele! 😄) in 2007 and gained quite a bit of traction since the very beginning.
In addition to the plain downloading capabilities, BitLet supports streaming of music and video content over bittorrent and uses UPnP to manage port mappings. I later wrote an extension for Google Chrome that simplifies the use of BitLet. We recently open sourced BitLet’s core code, the UPnP support library and the BitLet for Google Chrome extension.