Download It!: FoxTorrent (Win, Mac, Ubuntu)
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.
[Edit] And I hear TorrentSpy.com is a good place to start. [/Edit]