Amulet Project: C++ successor to Garnet user interface development environment; portable across MacOS, X11 (Unix, Linux), Windows 95/98/Me/NT/2000/XP. Descriptions, papers, documents, FAQs, links, comments. Carnegie Mellon University

FLTK: The Fast Light Toolkit is a cross-platform Graphical User Interface library written in C++. Site includes downloads, documentation, screenshots of widgets, developer information, and discussion forums. [LGPL

GTK+: GIMP Toolkit: Library to build graphical user interfaces (GUIs) originally for X Window. Designed to be small, efficient, flexible. Coded in C, in very object-oriented style. Language bindings: C++, Perl, Python, Objective-C, Guile. [Open Source, LGPL

wxWidgets: A cross-platform GUI and tools library for GTK, MS Windows, and MacOS. Uses native controls and emulates foreign functionality such as MDI, Tree Controls or Notebooks. Free, non-restrictive license