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.
PostgreSQL Database Server (http://www.postgresql.org/)
PostgreSQL is a powerful, open source relational database system. It has more than 15 years of active development and a proven architecture that has earned it a strong reputation for reliability, data integrity, and correctness. It runs on all major operating systems, including Linux, UNIX (AIX, BSD, HP-UX, SGI IRIX, Mac OS X, Solaris, Tru64), and Windows.
yet another forking process controller
PTFS/Liblime KOHA (https://github.com/liblime/LibLime-Koha)
Puck's Gnucash Tools (http://www.etc.gen.nz/projects/gnucash)
Tools for working with Gnucash. Such as loading txn information into a CalDAV calendar.
Puppet module for graphite monitoring tools
Modules for managing installation/configuration of OpenStack's identity management
Manipulate audio with a simple and easy high level interface