Add threaded comments to Rails in only 7 lines of code: has_threaded_comments (github.com)

This is actually my first serious open-source project. I have nominally released other code as open-source, but never in a way that made collaboration easy. The system itself is an extraction of the commenting system that I have been using in production on one application for over 6 months. I've learnt a lot by putting this together, hopefully other people find it useful too!