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.
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.
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.
Wt is a C++ library for developing Web applications with an API that is widget-centric and inspired by existing C++ graphical user interface APIs. To the developer, it offers abstraction of Web-specific implementation details, increasing the accessibility and portability. Under the hood, the library uses the latest techniques (HTML5, Ajax, WebSockets) to handle user events and update the Web page.
Panwa is a framework based on the Qt toolkit for writing platform independent applications with high quality graphical user interfaces. Panwa applications are meant as a complementary approach to Web applications, and the framework makes it possible to create applications that combine the flexibility of Web applications with the usability advantages of native applications. Therefore, you can see it as an implementation of a "rich thin client".