DuckDuckGo is partly closed source, but increasingly open source. If you want to contribute, a great place to get started is at DuckDuckHack, our open source platform for instant answers.
Please see the overview of our open source projects.
DuckDuckGo is also largely built with Free and Open Source Software (FOSS). That's why we give back FOSS -- check out our 2013, 2014, and 2015 donations.
Prior versions on GitHub.