I’m in the process of adding support for additional torrent indexers. For the time being Jackett is a torznab middle layer / proxy server that you install along side Sonarr. It must first be configured with logins/cookies from your torrent sites and Sonarr must be pointed at it as a Torznab indexer source.
I am in the process of developing support for BitMeTV (which Sonarr currently supports RSS sync but not search) and Freshon.tv. Feel free to leave requests for your favorite public or private torrent sites.
The project is in C# and will be uploaded to github when I have something working. It will support Windows by default, please comment if you would also like a package for your OS.
Perhaps at some point the project can be merged into Sonarr if the devs and community support the idea.
EDIT: ALMOST ready for testing, progress so far on github: https://github.com/zone117x/Jackett
EDIT 2: Initial alpha release available for download: https://github.com/zone117x/Jackett/releases
EDIT 3: Now tested and working on Linux, and should work on any other system with mono