libTISCH is a framework for easy development of multi-touch applications. It contains tools for video processing, calibration, gesture recognition and, most importantly, a GUI toolkit with widgets that utilize the multi-touch gestures detected by the lower layers. It runs on Linux, Mac OS X, and Windows.
QtAda is an Ada2005 language binding to the Qt libraries and a set of useful tools. It allows you to easily create powerful cross-platform graphical user interfaces completely on Ada 2005. QtAda applications use the native look and feel on every supported platform. It also allows you to develop your own widgets and integrate them into the Qt Designer for high speed visual GUI development. It uses a native thread-safe signal/slot mechanism and provides full transparent integration with Ada tasks.
The Ecere SDK is a cross-platform toolkit for building software applications. It currently runs on Windows, Linux, Mac OS X (through X11), FreeBSD, and the Android OS. It should run on other Unix platforms with minor testing/tweaking. With the Ecere SDK, you can develop applications once and deploy them on all supported platforms alongside a lightweight runtime environment. It introduces eC, an object oriented language derived from and fully compatible with C, compromising neither runtime performance nor ease of use. A built-in 3D engine supporting both Direct3D and OpenGL is fully integrated.
Minimal OpenGL Utility Library (MOGUL) is a library similar to GLUT, only with fewer features. It lets you write window-system-independent OpenGL programs with even fewer lines of code than GLUT. It currently supports X11 and Win32. Examples are included in the distribution. It is intended to be easy to learn and simple to use.
Faith is a cross-platform C++ graphical application toolkit. It runs natively on Linux/Unix (Xlib) and Windows. It has an API strongly inspired by Qt's and has many features that are expected from a modern toolkit, including a variety of common "widgets", an aesthetically pleasing appearance, and the many critical non-portable components such as anti-aliasing and sockets.