This is a list of notable software forks.
Undated
The many varieties of proprietary Unix in the 1980s and 1990s — almost all derived from AT&T Unix under licence and all called "Unix", but increasingly mutually incompatible. See UNIX wars. Most Linux distributions are descended from other distributions, most being traceable back to Debian, Red Hat or Softlanding Linux System (see image right). Since most of the content of a distribution is free and open source software, ideas and software interchange freely as is useful to the individual distribution. Merges (e.g., United Linux or Mandriva) are rare. Pretty Good Privacy, forked outside of the United States to free it from restrictive US laws on the exportation of cryptographic software. The game NetHack has spawned a number of variants using the original code, notably Slash'EM (1997), and was itself a fork (1987) of Hack. Openswan and strongSwan, from the discontinued FreeS/WAN.
1981
Symbolics Lisp Machine operating system, later called Symbolics Genera. Forked from the MIT Lisp Machine operating system, which was licensed by MIT to Symbolics in 1980. This fork later motivated Richard Stallman to start the GNU Project.
1985
POSTGRES (later PostgreSQL), after Ingres branched off as a proprietary project.
1990
Microsoft SQL Server, from Sybase SQL Server, via a technology-sharing agreement concerning the Tabular Data Stream protocol. SWLPC, from LPMud.
1991
Xemacs, from GNU Emacs, originally for Lucid Corporation internal needs.
1993
FreeBSD, started as a patchkit to 386BSD. NetBSD, started as a patchkit to 386BSD.
1995
Apache HTTP Server, from the moribund NCSA HTTPd. OpenBSD, a fork of NetBSD 1.0 by Theo de Raadt due to internal developer personality clashes.
1997
EGCS was a fork of GCC, later named as the official version.
1998
Grace, from Xmgr, after that project ceased development.
1999
FilmGIMP, later called CinePaint, from GIMP, to handle 48-bit colour. OSSH from SSH, when that project was proprietised. OpenSSH, from OSSH. Sodipodi, from Gill. Steel Bank Common Lisp, from CMU Common Lisp.
2000
TrueCrypt, from E4M when the latter was discontinued. Tux Racer went proprietary in 2000, leading to several forks including OpenRacer, PlanetPenguin Racer and Extreme Tux Racer. OpenOffice.org, from StarOffice after Sun Microsystems made the source code publicly available. OpenOffice.org was eventually forked into LibreOffice.
2001
ELinks, began as an experimental fork of Links. Fluxbox, from Blackbox. GNU Radio, from pSpectra. Xvid, was a fork of OpenDivX. WebKit, project was started within Apple by Lisa Melton on 25 June 2001 as a fork of KHTML.
2002
GForge, from SourceForge. GraphicsMagick, from ImageMagick due to concerns over the openness of development. The Matroska container format, from the Multimedia Container Format, due to differences in direction. MirOS BSD, from OpenBSD. Syllable Desktop, from the stagnant AtheOS.
2003
aMule, from xMule, which itself forked from lMule shortly before, over developer disagreements. b2evolution, from b2/CafeLog. DragonFly BSD, from FreeBSD 4.8 by long-time FreeBSD developer Matt Dillon, due to disagreement over FreeBSD 5's technical direction. Epiphany, from Galeon, after developer disagreements about Galeon's growing complexity. Inkscape (vector-graphics program), from Sodipodi. NeoOffice, a fork of OpenOffice.org, with an incompatible license (GPL rather than LGPL), due to disagreements about licensing and about the best method to port OpenOffice.org to Mac OS X. The Safari renderer that became WebKit, from KHTML. sK1, from Skencil when the latter moved from Tk to GTK+. WordPress, from b2/CafeLog. Zen Cart, from osCommerce.
2004
Baz, the previous version of Bazaar, from GNU arch. FrostWire, from LimeWire after LimeWire's developers considered adding RIAA-sponsored blocking code. MediaPortal, from XBMC. WineX (later Cedega), was a proprietary fork of Wine. XOrg, from XFree86, in order to adopt a more open development model and due to concerns over the latter's change to a license many distributors found unacceptable.
2005
Audacious, from Beep Media Player to continue work on the old version of that project. Joomla, from Mambo due to concerns over project structure. Claws Mail, from Sylpheed, due to perceived slowness in accepting enhancements.