XTracker is a Facebook application that tracks and keeps a record of all your unfriended contacts. What sets this apart from other similar applications is that it uses an elegant clean UI, has no ads, supports Facebook Secure Browsing (SSL), and provides a "reunification feature" which will inform you if a previously unfriended contact has re-added you or re-activated his/her account.
Written in Ruby on Rails using Koala library. It uses IFRAME for canvas and signed_request to pass data.
See it live at http://apps.facebook.com/_xtracker
This was my very first Ruby on Rails project, so don't expect the code to be optimized. Refactoring this code is definetely on my TODO list.