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.
Drupal (core) (http://drupal.org/)
Drupal is an open source content management platform, allowing an individual or a community of users to easily publish, manage and organize a great variety of content on a website. Equipped with a powerful blend of features, Drupal is able to support community sites, blogs, portals and more. This is Drupal's core, you can find the contributed modules page at http://ohloh.net/projects/3502.
The Drupal Console is a command line interface tool, to generate boilerplate code and interact with a Drupal 8 installation.
PHP library to embed any web service or page (oembed, opengraph, etc)
Perl CPAN module for fixing up data which contains mixed encodings (ASCII / UTF-8 / latin-1 / CP1252) and outputs clean UTF-8
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.
FAI is an automated installation tool to install or deploy Debian GNU/Linux and similar distributions on a Linux Cluster or a bunch of different hosts. It's similar but more flexible than other tools like kickstart for Red Hat, autoyast and alice for SuSE or Jumpstart for SUN Solaris.
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