Checklister
At Grace Hopper 2009, I went to the code sprint hosted by the group Systers-dev. In free times from 2009 to 2010, I developed an open-source checklist for them. It can be used to create checklists to ensure that functionality isn't impaired when new features or bug fixes are added.
Technical Details
Set up
It is a PHP-MySQL web application (and is also one of the last I developed, as Django makes things like this a lot easier).