X - a portable, network-transparent window system
The X Window System is a network transparent window system which runs on a wide range of computing and graphics machines. It should be relatively straightforward to build the X Consortium software distribution on most ANSI C and POSIX compliant systems. Commercial implementations are also available for a wide range of platforms.
XFree86 is a freely redistributable open source implementation of the X Window System.
Another implementation is XOrg: