XUL - XML User Interface Language.
The UI language designed for and used in Mozilla
Glade
A user interface builder that outputs XML interface description files which can be be processed by several back-ends
AAIML - Alternate Abstract Interface Markup Language
AUIML - Abstract User Interface Markup Language
XIML - eXtensible Interface Markup Language
UIML - User Interface Markup Language
Implementations
XForms - W3C Web Forms Specification
WML
XwingML