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.
The official Linux kernel from Xilinx
Logstalgia (aka ApachePong) replays or streams a standard website access log as a retro arcade game-like simulation.
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 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