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.
A silverstripe form field based on http://ivaynberg.github.com/select2/
A SilverStripe module for creating templates within the CMS that can override those defined in the theme, for both Layout and main templates
Faster, easier client and server-side form validation for SilverStripe
simple chaining hash table implementation written in pure C (c99)
Kernel for I9300
Database management tools from Skype: WAL shipping, queueing, replication. The tools are named walmgr, PgQ and Londiste, respectively
A collection of snippets for snipmate
Snowy is a web application for synchronizing, viewing, sharing, and editing your Tomboy notes online. It is designed to power an upcoming Tomboy Online free web service where any Tomboy user can make an account. It can also be used on your own personal server. Snowy is AGPL-licensed and written in Python using the Django framework.
SOFA Statistics (http://sofastatistics.com)
SOFA is a statistics, analysis, and reporting program with an emphasis on ease of use, learn as you go, and beautiful output.