http://www.gimp.org/

http://www.itc.virginia.edu/desktop/mac/osx_intro/osxintro.html

http://en.wikipedia.org/wiki/GUI

http://en.wikipedia.org/wiki/

http://en.wikipedia.org/wiki/Alan_Kay

http://en.wikipedia.org/wiki/Computer_software

http://en.wikipedia.org/wiki/System_software

http://en.wikipedia.org/wiki/Free_software