PyMaTi is a simple and easy to use GUI for numerical and scientific computing in Python. It surrounds well know packages NumPy and Matplotlib and provides possibility to immediately play with numerical python from intuitive user interface.
HoloMenu is a C library for creating GUIs with holographic simulator view. It helps creating buttons, text boxes and other components with effects similar to Syndicate Wars. It is designed to use as menu in games, but may be used for other projects.
A free rendered GUI library providing windowing and widgets for the easy creation of HUDs for games. It supports themes/skinning, XML-based configuration, and works with any graphics API or rendering engine including Axiom, Irrlicht, and SDL.NET.