It's called BitLet 4 Chrome and you can find it on Chrome's Extension Gallery.
Here's how it works:
The BitLet 4 Chrome extension makes it easier to download files with bittorrent using your chrome browser.
Once installed, your browser will detect all the links to torrent files within the pages you're browsing and it will make those links open BitLet's download popup, allowing you to start downloading in a few clicks.
The extension itself is Open Source: you'll find the source code at http://github.com/abahgat/BitLet4Chrome.