Download It!: FoxTorrent (Win, Mac, Ubuntu)
An extension for Firefox, FoxTorrent is a very, very practical BitTorrent client --working right within your browser.
The open source FoxTorrent Firefox extension that lets you stream torrents as they download, from your web browser, with zero configuration needed.
I've been using it for about a week, and my only complaint is that you can't choose the download folder (currently, it's set to download files into the FoxTorrent folder, rather than whatever you set Firefox's default download folder to be). Other than that, I love it.
So, in the key of Fergie, I urge you all to check it out!
[Edit] And I hear TorrentSpy.com is a good place to start. [/Edit]