X.25
X3D
X.400
X.500
X.509
XACML
XAML
XBEL
XDR
xDSL
Xerox Corp.
XForms
XHTML
XHTML basic
XKMS
XLink
XMI
XML
XML-Schema
XML-Signature
X-Modem
XNS
XON/XOFF
XPath
XQuery
XrML
XSL
XSL-FO
XSLT
XSV
X-WiN