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.
MediaWiki extensions (parent) (https://www.mediawiki.org/wiki/Category:Extensions)
A parent repository containing all the MediaWiki extensions of the Wikimedia repository as submodules (the list of submodules is semi-automatically updated and is usually complete). For a more granular classification of the individual extension repositories, see: https://www.ohloh.net/p/mediawiki-extensions-wmf https://www.ohloh.net/p/mediawiki-extensions-wmf-hosted
melo's cil (http://kapiti.geek.nz/software/cil.html)
DVCS-powered ticketing system
Mixxx is an open source DJ tool designed for both professional and amateur DJs alike. Mixxx allows DJs to mix music live with a clean, simple interface. Futhermore, Mixxx has a number of key features to help DJs in the mix: Beat estimation, parallel visual displays, and support for various DJ hardware controllers. Mixxx is available on all major desktop operating systems, including Windows, Mac OS X, and Linux.
Access the Libravatar API in Mojolicious
Moodle 2.x My Courses block - an alternative to the Course Overview block used on "My" Homepages.
Auto enrol plugin for Moodle
Moodle (lang) (http://docs.moodle.org/en/Translation)
Moodle is translated into over 70 languages by a dedicated corps of worldwide volunteers. Translation has actually been going on a lot longer than the stats here may suggest, because the language packs used to be part of the main Moodle code base.
New version of the Attendance module for Moodle 2.4 and higher. (replaces mod_attforblock)