X11

One of the most popular implementations of the X-Window system, X11 is supported by the Open Group X Project team.

X.25

A packet-switched data network.

X.400

A CCITT standard for mail formats.

X.400 A communications network tool. X.400 defines a message transfer protocol with advanced access control and accounting features

X.500

A directory system, X.500 is being superseded by more modern LDAP systems.

X-Modem

One of the earliest reliable file transfer protocols, Xmodem was written in 1977 by Ward Christiansen for use on the first BBS.
See also Ymodem and Zmodem.

X-Window

A network-based GUI designed for Unix systems, there are thousands of free applications available as source code and compiled executables for X-Window compatible systems, including PC-based X-terminal emulators. X-Windowing systems are interesting in that they reverse the usual client- server metaphor.