Magnetise is a simple extension for Apple's Safari browser which allows BitTorrent downloads to be started directly from a textual info hash on a web page.
An info hash is a 20-digit hexadecimal string (usually) which identifies the contents of a Torrent. A BitTorrent client can use this to find and download the content. For example, let's suppose we're interested in getting Ubuntu ISO images. A quick Google will lead us to the following page:
You can see the info hashes for the various different torrents listed in the first column. Now if you bring up the context menu on one of these strings you will see an option Magnetise. Selecting this will open an ephemeral tab in the browser, launch the BitTorrent client (for example, μTorrent) to fetch the torrent, and then close the ephemeral tab:
Note: the Magnetise option does not appear if the context is not a 20-digit hexadecimal string.
Often, when you are searching for some content, you can find its info hash simply by searching on Google for something like "blah blah blah info hash", and in the search results you will see the info hashes. Thus, you can launch a torrent download directly from the Google search results page, without visitng any websites which may be associated with nefarious activity, may host malware, etc.
Magnetise with an 's' not a 'z' is the standard British English spelling of the word, by the way.
You can download the current version (1.4) of the extension here.