TorrentSpy can relate to two different things: A now defunct BitTorrent indexer, which was the most visited in 2006 according to TorrentFreak, and a little program from 2004 to extract information from a torrent-file, showing users everything from the files included to the current ratio of seeders and peers. You can find the official website here.
(TorrentSpy in action)
Can Be Of Use For Very Old Torrents
Back in the days, the information on some BitTorrent indexers were sparce, and people were in bad need for a software that showed them more information. For example, it can save you from downloading a “dead” torrent, where the file is not available 100% on the network anymore. Find a list of all features here.
Today, it might still be useful if you happen to find an old torrent on your hard drive and wonder how it is doing, or in the very rare case you find a torrent somewhere without any updated stats. However, consider that TorrentSpy can’t see if, maybe, potential sources are available on DHT as it only supports trackers.
All in all, this little piece of software may be of use for enthusiasts that love to see what information is contained in a torrent, and in some rare cases also for normal users, but mostly it is a relict from times long gone.