Firefox: If you've used a BitTorrent search engine that lists lots of torrents but doesn't actually host any torrent files (like Torrentz), most of the time you still have to go to other third-party sites for the download. Not so with Magnetiser.
When you search a torrent aggregation site like Torrentz, all you see are links to external sites hosting the torrent file. Most of the time that means you'll need to click through to other sites to hunt for the download link to the torrent you're looking for. However, BitTorrent clients like uTorrent support magnet links, which for the purpose of BitTorrent, allow users to start downloads without an actual torrent file.
Once installed, Firefox extension Magnetiser monitors your searches on Torrentz (the largest BitTorrent search engine that doesn't host torrent files) or isoHunt. If it detects that you're on a torrent's detail page, you'll see "Magnetise" in the Firefox status bar. Click on Magnetise to open the torrent in your default BitTorrent client.
If you're not sure when this could come in useful, all-things-BitTorrent weblog TorrentFreak makes a good case: "Not only does it save time, it also comes in handy when a torrent is no longer available on the external site." It's also useful for people with ISPs or IT people who block .torrent file downloads.
Magnetise hasn't yet been approved at Mozilla's Add-on site, but you can download the magnetiser.zip, change the file extension from ZIP to XPI, then drag and drop the XPI into Firefox. (A bit of a hassle, we know. When it is approved, it should be available with a click-to-install here.)
Have you ever used magnet links for your BitTorrent downloads? Let's hear about it in the comments.