Ones that come to mind right away and support most popular OSs: 1) wxWidgets 2) QT 3) Delphi with Firemonkey 4) Lazarus Whole bunch of other which you are

Ones that come to mind right away and support most popular OSs: 1) wxWidgets 2) QT 3) Delphi with Firemonkey 4) Lazarus Whole bunch of other which you are welcome to Google and explore Also it depends on your type of application but I am seriously considering using 3D rendering library as a basis for multiplatform GUI. This one does not look too native but it looks like it may be the leanest, fastest option. This would require some groundwork first.