Open Source Rails: An Index of Open Sourced Rails Apps
Open Source Rails presents profiles of open sourced Ruby on Rails apps. The site allows you to submit application of your own, comment on existing apps, update the details for other apps, and keep track of those you're interested in. On most profiles you're given direct links to the app's homepage, main developer(s), and source code repositories, making it really easy to get involved.
As fits the name of the site, the system powering Open Source Rails is open source itself!