railscasts.com in open source (outdated).
the best open source ruby on rails (or rack) web applications! Submit issues and pull requests to add or update projects on the gallery.
Open Source School/Campus Management Rails Application.
A free open source social network based on privacy
Yet another Open Source Social Network on Ruby on Rails
An example of a Rails API made with Grape.
3rd-party-retargeter is a self-hosted Ruby on Rails application that allows you to add a retargeting pixel to ANY of your tracking links whether you own the website it points to or not.