> Something else? GTK+ still doesn't look very native on Windows or OS X, so for many cross-platform apps it's not a viable option.
GTK+ can look just like Windows XP, Windows 7, Mac OS X. It has themeing capability.
GTK+ can look just like Windows XP, Windows 7, Mac OS X. It has themeing capability.
Qt technically doesn't use the native UI widgets either, but it does a much, much better job at emulating them (and the visual side is completely flawless at least on Windows). In addition, it actually proxies to native functionality where available, which means that you get the real file picker of your platform, dialogs have the correct button order, etc.
GTK+ is only good for GNOME. This is why I wish that Qt would become the de facto standard toolkit for Linux developers: it fits in perfectly in both KDE and GNOME. (Actual KDE apps that use their libraries are a different story, though.)
I've never had anything to complain about the UI of Qt apps on any of the desktop platforms I've used them on. Qt is the best choice for cross-platform UI.