great cross platform GUI API with alot of commercial/opensource applications. its GPL licence that allows you to have private sources if you dynamically link to the library.
plasma desktop is shaping up very nicely aswell, its compiled using Qt.
qtCreator – cross platform IDE works best on OSX to reach all target systems including IOS,android, windows. for linux you’ll need a virtualmachine and something like xhyve/qemu/virtualbox virtualmachine
google makes a great way to find code here are a few searches of mine