I have been involved with Network UPS Tools, an open-source UPS monitoring system, for about ten years. What started as a simple patch for one of the serial UPS drivers turned into a full-fledged software engineering experience, including setting up a continuous integration system with Buildbot, and a migration to Subversion, and subsequently, Git (via Eric S. Raymond's reposurgeon).
Source code is hosted at GitHub. Occasionally, I will post test code to my own Github repository, but that usually gets merged after testing.