XEmacs was a fork of GNU Emacs, driven by technical and philosophical disagreements with RMS. For a while it was rather popular, but then it died. Its failure is a counterargument to these complaints about RMS' management style–if his decisions are really as bad as some people say, why didn't the fork succeed? And why won't any of the complainers start up a new fork themselves, or revive the moribund XEmacs fork?

You might be thinking of GCC – the current GCC is descended from a fork of GCC (EGCS). RMS agreed to let the forkers take over the official project.

GNU Emacs itself is a fork of Gosling Emacs. The original Emacs was written in TECO, a rather horrendous language. Soon, people started writing Emacs clones in Lisp – Multics Emacs, and some of the Lisp Machine editors. James Gosling wrote the first Emacs for Unix platforms; for an extension language, he invented "Mocklisp", which resembled Lisp on the surface, but lacked its power and generality. GNU Emacs started out as a fork of Gosling Emacs, adding a real Lisp instead of Mocklisp. Gosling made his Emacs freely available under an unclear license, and then sold it to a commercial firm and declared its original free redistribution rescinded. To get away from that legal problem, RMS rewrote all the remaining Gosling Emacs code.