Now that Qt has been released under GPL on Windows (link provided by glazblog (fr)), it runs freely on UNIX/X11, MacOS X and Windows. But where is Gtk ? Gtk-2.x runs on UNIX, on Windows, but still require X11 on MacOS X not making it really friendly. There is a port of Gtk+1.2 on MacOS X using Carbon done by the CinePaint team, but this is Gtk+1.2 while everything is currently using some 2.x version; and the project seems to be dormant.

I don't understand the lack of interest for this port by the MacOS X community. They are the first to whine about The Gimp being not well integrated with MacOS X, but porting Gtk+2.x to MacOS X without using X11 would probably resolve most of the issues. And it would also resolve the issue of lack of decent not overpriced and/or closed spreadsheet by allowing to port Gnumeric. This lack of interest is still visible with the lack of developers for AbiWord on MacOS X. When I had to stop developing it due to lack of hardware, nobody did anything until FJF took over. And it is not the lack of interest from people, it is just the lack of skilled MacOS X developers willing to work on Open Source projects.