reddit
The code that powers reddit.com.
Project Source: https://github.com/reddit/reddit
Project Homepage: http://www.reddit.com/r/redditdev

readthedocs.org The source code to readthedocs.org.
Project Source: https://github.com/rtfd/readthedocs.org
Project Homepage: https://readthedocs.org/

pinry
The open-source core of Pinry, a tiling image board system for people who want to save, tag, and share images, videos and webpages in an easy to skim through format.
Project Source: https://github.com/pinry/pinry
Project Homepage: http://getpinry.com/

class2go
Class2Go is Stanford's internal open-source platform for on-line education.
Project source: https://github.com/Stanford-Online/class2go
Project Homepage: http://class2go.stanford.edu/

pythondotorg
The new python.org.
Project Source: https://github.com/python/pythondotorg

zamboni
The backend of the Firefox Marketplace.
Project Source: https://github.com/mozilla/zamboni
Project Documentation: http://zamboni.readthedocs.org/en/latest/