TraktRater is a tool written in C# to help users transfer user episode, show and movie user ratings and watchlists from multiple media database sites around the web.
The Letterboxd API is in private beta. If you’d like to be considered for early access, please email details of your intended use to api@letterboxd.com. Please include the title of your app or project in the subject line, and note that while we read all applications, we are unable to individually reply, or to guarantee admission to the beta.
If you require your account data in machine-readable form we have import and export options, and every account has an RSS feed of new diary entries and lists.
https://letterboxd.com/api-beta/