Lumen is a GUI library for Ada. It uses OpenGL for all its display functions, so it lends itself naturally to visual applications such as simulations, games and visualization.
After having been in hibernation for a while, development is starting to pick up again. Here are some interesting links:
Lumen is a very interesting project. If you have questions, or are interested in helping further Lumen, then all you have to do is connect to the #ada IRC channel at freenode.net and ask. The developers are usually online.