The open source movement is an offshoot of the free software movement
that advocates the term "open-source software" as an alternative label for free software
(libre definition), and primarily makes its arguments on pragmatic rather than philosophical grounds.
Nearly all open source software is also "Free Software" (when the "libre" sense of Free software is meant).
Open Source Software is typically abbreviated as "OSS", since "OS" is a common
abbreviation for "operating system".
A popular rallying point for the OSS community is SourceForge, as it provides free web hosting and other services to open source projects.