I've been using EGUI to write GUI programs. Works nicely and runs on both Linux and Windows without notable differences. (I haven't tried Mac, but it presumably

I've been using EGUI to write GUI programs. Works nicely and runs on both Linux and Windows without notable differences. (I haven't tried Mac, but it presumably will work there as well without changes). Small binaries (a few Mb), and can integrate with maps, 3D rendering etc.