These are some of the public projects that uses libtorrent. If you want your project listed here, let me know.
electric sheep
electric sheep is a screensaver which collectively generates animations and lets the users vote which one to live on.
qBittorrent
qBittorrent is a QT bittorrent client available for linux (likely portable to most other desktops as well). Written by Christophe Dumez.
btg
btg is a linux bittorrent client which is run as a daemon. It has multiple user interfaces which connects to the daemon. One GUI (Gtkmm), one terminal interface (ncurses) and one web interface (accessable through a web browser). Written by Michael Wojciechowski and Johan Ström.
MooPolice
MooPolice is a windows bittorrent client with a quite unique look.
BitsCast
RSS reader, podcast subscriber, video feed downloader, bittorrent feed downloader. See BitsCast homepage for more info.
BitSlug
BitSlug is a MacOSX cocoa client.
BitBuddy
BitBuddy is a windows bittorrent client by Daniel Wang, quite similar to BitComet GUI-wise.
Arctic Torrent
Arctic Torrent is a light-weight bittorrent client for windows. Written by Cory Nelson.
ZipTorrent
ZipTorrent is another windows bittorrent client with decentralized search facility.
FireANT
FireANT is a video blog available for Windows and Mac OSX.
eDonkey 2000
eDonkey2000 comes bundled with a bittorrent plugin by 'The Orphan'.
pHoeniX
pHoeniX is an eMule mod supporting, among other things, bittorrent.