SourceForge is a collaborative software development management system. SourceForge software is sold by VA Software. It provides a front-end to a range of services and integrates a number of open source applications (such as GNU Mailman and CVS).
SourceForge.net is a centralized location for software developers to control and manage open source software development. SourceForge.net is hosted by VA Software and runs a version of the SourceForge software. A large number of open source projects are hosted on the site, although it does contain a lot of dormant or single user projects.
The Free Software Foundation created their Savannah development site as a response to the perceived proprietrisation (closed source state) of the SourceForge software. Several collaborative sites now exist that utilise the SourceForge software.
Formerly the Sourceforge code was publicly available, but one day they decided to close the code. Last officially available code was 2.5, and the CVS version just before closing was 2.61. Savannah is a derived work based on the 2.0 version.
Gforge.org: One of the main Sourceforge programmers started a significant rewrite—the Gforge project—based on the last known CVS version of Sourceforge. New features have been added and free development continues there.