portable C++ GUI library designed for games using Allegro, SDL and/or OpenGL, comes with a standard set of widgets and can use several different backends for displaying graphics and grabbing user input
Hardware accelerated 2D full featured middleware for all who want to develop commercial quality 2D games rapidly and easily. It covers all imaginable 2D game genres (DirectX)
a free library providing windowing and widgets for graphics APIs / engines where such functionality is not natively available, or severely lacking. The library is object orientated, written in C++, and targeted at games developers who should be spending t