GNU is a computer operating system composed entirely of free software. According to wikipedia:
The plan for the GNU operating system was publicly announced on September 27, 1983, on the net.unix-wizards and net.usoft newsgroups by Richard Stallman. Software development began on January 5, 1984, when Stallman quit his job at Massachusetts Institute of Technology so that they could not claim ownership or interfere with distributing GNU as free software.
FSF has started its month long celebration of the anniversary by publishing Happy Birthday to GNU film:
(Video.01: Freedom Fry — "Happy birthday to GNU" - short film by Stephen Fry)
I am looking forward to another 25 years!