X-1
Xbase
XBASIC
XBL
XC
XDL
Xduce
XFDL
Xfun
XHTML
Xi
XICS
XIDL
XL
XLink
XLISP
XLISP-PLUS
XLL
XML[1]
XML[2]
XML-GL
XML-QL
XNF
Xpath
XPC
XPL
XPointer
XPOP
Xscheme
XSL
XSLT
XTRAN
XUL