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.
Mahara Scripts (https://git.mahara.org/scripts/mahara-scripts)
Scripts for the Mahara project https://mahara.org
Map of CPAN (http://mapofcpan.org/)
The map of CPAN (http://mapofcpan.org/) is a fun way to visualise and explore the universe of Perl modules uploaded to CPAN.
MariaDB is one of the most popular database servers in the world. It’s made by the original developers of MySQL and guaranteed to stay open source.
MediaWiki extension: CirrusSearch (https://www.mediawiki.org/wiki/Extension:CirrusSearch)
The CirrusSearch extension implements searching for MediaWiki using Elasticsearch.
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
MediaWiki extensions hosted by WMF
Umbrella of MediaWiki extensions hosted in the Wikimedia Foundation servers but not deployed in official Wikimedia projects. See also the umbrella for extensions supported by the WMF at https://www.ohloh.net/p/mediawiki-extensions-wmf
Medusa is a webservice API for WRMS (Work Request Management 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 (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.