Rapicorn is a graphical user interface toolkit for rapid development of user interfaces for C++11 and Python development. The source code is released as Free Software and everybody is welcome to contribute ideas, code, wiki contributions and artwork.
FOX is a C++ framework for graphical user interfaces. It provides an effective and simple API, traditional but platform-agnostic themes, many common widgets and controls, drag'n'drop support, selections, icons/images, tooltips and hover widgets, but also 3D features per OpenGL. Its speed comes from conservative memory management, spatial redrawing, and bi-directinal event/message passing between GUI elements.
Eagle Mode is a C++ GUI toolkit and provides a zoomable user interface (ZUI) usable as desktop shell even. It functions as filesystem browser and manager, and seamlessly runs integrated applications. Images, text files and office documents can be zoomed in, audio and video files engaged as well. Built-in widgets like an archiver, chess and 3D minesweeper games, a clock, fractal imaging examples integrate likewise.
FLTK (fulltick) is a lean but modern GUI toolkit written in C++ with bindings for Perl, Python, Ruby, Lua and Tcl. It's usable and looks alike on all platforms, provides full internationalization, 3D rendering via OpenGL and an internal GLUT backend, static linking, does not require macros or preprocDessing, still comes with a solid set of widgets, and its GUI designer FLUID.
Nux is a OpenGL-based widget toolkit, with some functional resemblence to Gtk+. It's developed and used by Ubuntus Unity. It's comprised of three libraries: NuxCore for types/math/color/urility functions, NuxGraphics abstracts the rendering and canvas interaction, and Nux itself providing the widgets atop.