wxWidgets lets developers create applications for Win32, Mac OS X, GTK+, X11, Motif, WinCE, and more using one codebase. It can be used from languages such as C++, Python, Perl, and C#/.NET. wxWidgets applications look and feel native.
GtkRadiant is a level design program developed by id Software and Loki Software. It is used to create maps for a number of computer games. It is maintained by id Software together with a number of volunteers.