Wikimedia Foundation

Preferred license: Primarily GPL; some related projects are MIT-style.
The Wikimedia Foundation, Inc. is an international non-profit organization dedicated to encouraging the growth, development and distribution of free, multilingual content, and to providing the full content of these wiki-based projects to the public free of charge. The Wikimedia Foundation operates some of the largest collaboratively-edited reference projects in the world, including Wikipedia, often measured as one of the 10 most visited websites.
Actual source code produced by the student participants in Google Summer of Code™ for Wikimedia Foundation can be found here.
 
Current Projects
by Michael S. Baynton, mentored by Brion Vibber