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.
Koha QA Tools
This is a repository containing scripts useful for release management/qa with Koha
Kobli is the result of adding to Koha some of the features suggested in the Evaluation Report of the Working Group of BAGEs (Libraries of the Spanish Central Government), some of which are already included in the Koha community.
LHC Interface MTF Offline (http://svnweb.cern.ch/guest/limo/)
XForms interface for engineers to fill in LHC installation data without an Internet connection.
LimeSurvey (previously known as PHPSurveyor) is a PHP web application that interacts with MySQL/MSSQL/Postgres to develop, publish and collect responses to surveys. There are several ways to insert data into the survey: either by a 'pretty' public screen which presents the question(s), or using a data entry screen. It includes the capacity to generate individual 'tokens' and branching and has numerous question types and basic statistics.
Linux Kernel (http://www.kernel.org/)
The Linux kernel is a Unix-like computer operating system kernel. It is used world-wide: the Linux operating system is based on it and deployed on both traditional computer systems such as personal computers and servers, usually in the form of Linux distributions, and on various embedded devices such as routers, wireless access points, PBXes, set-top boxes, FTA receivers, smart TVs, PVRs and NAS appliances. The Android operating system for tablet computers, smartphones and smartwatches is also based atop the Linux kernel. (from Wikipedia)
Linux NTFS file system support (http://www.linux-ntfs.org/)
The goals of this project are: create a new Linux kernel driver for the NTFS file system (v1.2 and later 3.0), user space utilities (e.g. format, ntfs check, etc.) and a library to avoid code duplication and provide access to NTFS to other GPLed programs
Logstalgia (aka ApachePong) replays or streams a standard website access log as a retro arcade game-like simulation.