Visit iknowwhatyoudownload.com and you’ll immediately see a list of torrents associated with your computer’s IP address. To look up a different IP, paste it into the search box and click Find IP.
The site’s second tab (Track Downloads) unlocks an even more interesting feature. Take any URL, paste it into the input field, and click Transform. In response, the service will give you a short link generated with goo.gl.
Now send it to someone and wait for them to click. If everything worked, you’ll see their IP address and a list of the torrents they’ve downloaded.
The short link still resolves to the URL you entered initially, so your friend probably won’t suspect anything. On the way, they’ll be briefly redirected through iknowwhatyoudownload.com and then sent to the final destination.
The service is far from perfect: it monitors about 500,000 torrents and collects data on 700 million hosts daily, but that’s still nowhere near enough to catch every BitTorrent user in the act. If you check a dynamic IP address, or it turns out not to be an end host but a proxy server, the torrent list may be full of junk—or empty altogether.
It looks like the site’s owners plan to monetize by offering API access to their database: they mention this option and advise interested parties to request access privately via email.
At the time of writing, I Know What You Downloaded was slow and periodically throwing 502 errors. It seems more people tried to poke at the new service than it could handle.