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.
melo's cil (http://kapiti.geek.nz/software/cil.html)
DVCS-powered ticketing system
Moodle 2.x My Courses block - an alternative to the Course Overview block used on "My" Homepages.
Auto enrol plugin for Moodle
New version of the Attendance module for Moodle 2.4 and higher. (replaces mod_attforblock)
the certificate module for Moodle
Dialogue Module for Moodle