FSF or The Free Software Foundation is a non-profit founded by Richard Stallman for the promotion of open source software and the management of GNU project issues.
The FSF is over 20 years old, and at one point employed software engineers to work on free software, however today the FSF is largely a legal organization, working on projects such as the GPL and LGPL.
These licenses are arguably the most popular free software licenses in the world, and the documentation license GFDL is used by Wikipedia, the largest encyclopedia in the world.