From Drupal and Moodle, right through to a Git versioning system and Perl modules on CPAN, we contribute back to over 175 community open source projects. This list is in order of the number of projects contributed to.
CPAN module: C/XS implementation of Encoding::FixLatin
Epiphany extensions (http://www.gnome.org/projects/epiphany/extensions)
Epiphany Extensions is a collection of extensions for Epiphany, the GNOME web browser.
Evolution provides integrated mail, addressbook and calendaring functionality to users of freedesktop.org compliant desktop environments.
Fez is an open source project to produce and maintain a highly flexible web interface to FEDORA for any Library or Institution to configure and publish or archive documents of any type sustainably.
Aaron Barnes' effort at tuning software for the FreeEMS… Written in Python using wxPython
Friends of SilverStripe modules
GStreamer SDK (http://www.gstreamer.com/)
Facilitates the use of GStreamer by providing an stable version of the framework, sample code and documentation. The GStreamer SDK is a joint project between Collabora and Fluendo
GitStats is a statistics generator for git repositories. It examines the repository and produces some interesting statistics from the history. Currently it outputs only HTML.