Gnome is a popular windows application framework built on top of X-windows. It provides an open-source API for developing Gnome applications. Gnome seeks to create an entirely GPL windowing environment and refuses to include "other-licensed" software into their distribution.
Gnome attempts to emulate object-oriented techniques using a standard C API. They provide extensive developer documentation including tutorials for people who are just starting to learn Gnome.
The library contains a complete "widget library" or set of user controls such as: buttons, text inputs, progress bars, scroll bars and drop-lists. Along with this, they provide containers for grouping sets of controls. The library even contains a "Multiple Document Interface (MDI)" that allows users to create complex applications.