I must strongly recommend against using Gtk+ as a GUI toolkit for any serious application except on GNU/Linux. The biggest reason for me is that Gtk+ has zero a

I must strongly recommend against using Gtk+ as a GUI toolkit for any serious application except on GNU/Linux. The biggest reason for me is that Gtk+ has zero accessibility support on Windows or Mac. That is, it doesn't work with assistive technologies which people with some disabilities need, such as screen readers for blind people. If you develop an application using an inaccessible toolkit, you're erecting a barrier for some people who might need to use your app, particularly if it's going to be used in an educational or business setting. So, what should you use instead? Qt is better, but not great. It's still bad enough that TeamTalk, a voice chat app that's popular among blind people, has a cross-platform Qt app but also a "classic" MFC app for Windows. The latter used to be the official Windows client, but is now maintained just for blind users. wxWidgets is better in some ways, because it uses native controls on each platform. Except where it doesn't; for example, the data view control is non-native on Windows, whereas the list view control is non-native in the Mac and GTK ports. and wxHTML is non-native across the board. Still, with some pain, wxWidgets can be made to work for cross-platform accessible apps. SWT might be a good choice, if you can make it work for .NET/Mono using IKVM.NET. SWT is better than wx in that where it has to use non-native controls, it implements the platform accessibility APIs; it helps that SWT is backed by IBM, which has the resources to put into this kind of thing, and the incentive to do so in the form of sales to government agencies and the like.