The Free Software Foundation (FSF) is planning a month of events to celebrate 25 years of the GNU operating system.

GNU, which stands for 'GNU's Not Unix', was developed by Richard Stallman and others to be free software that could compete with proprietary operating systems.

Over 300 pieces of software for GNU have been developed to date.

The GNU Project has developed many applications and was fundamental in the creation of the GPL free software licensing agreement.

Peter Brown, FSF executive director, said: "We intend for the 25th anniversary to be more than just a reflection on the history of the free software movement.

"Despite all the success brought about by the GNU system and other free software projects, we still need a determined effort to replace or eliminate the proprietary applications, platforms, drivers and firmware that many users still run."

To kick off the celebrations actor Stephen Fry has taken part in a film extolling the virtues of free software and urging people to use it. The film is available to view or download on the GNU web site.

"Stephen has generously donated his time to the cause of free software," said Matt Lee, FSF campaigns manager and writer/producer of the film.

"His ability to communicate a technological and philosophical movement in terms of the basic principles of sharing and user freedom - ideas that everyone can understand - will introduce a new and broader audience to the benefits of free software."

The FSF is declaring 20 September as Software Freedom Day and planning an additional celebration on 27 September to coincide with the 25th anniversary of the day on which work started on GNU.