FrostWire, a BitTorrent client (formerly a Gnutella client), is a collaborative effort from hundreds of Open Source and freelance developers from around the world. It is a free, but ad-supported file downloading application that's well-designed and comprehensive.
Here are some screenshots to illustrate the features and user interface of this application:
Tixati
BitComet
qBittorrent Portable
Freenet
uTorrent