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.
XML::Simple is a Perl module that makes it really easy to read and write XML files. It was originally developed for the purpose of reading and writing config files in XML format, many people find it useful for other purposes.
Zaqar OpenStack Queuing and Notification Service (https://wiki.openstack.org/wiki/Marconi)
OpenStack Queuing and Notification Service
Zen Kernel for Galaxy Nexus variants (toroplus/toro/maguro)