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I suggest using simply because it's able to create an almost native look on almost any platform. For example, it's uses the global menu on Mac OS X and Unity (patches merged in 4.8) and inherits the GTK theme when using a GTK desktop environment.
One of the best examples of showing the native looks of a Qt4 application on any platform would probably be VirtualBox.
On Mac OS X:
You can see how the gradients, buttons, dialog boxes, tabs, etc. all look consistent with other Mac applications.
On KDE and GNOME/Unity:
You can see how VirtualBox integrates into the desktop environments and how it uses the native GTK file selection dialog when used on GNOME.
When you selecting your toolkit, I think you should find one that "fits" in your existing code. Since Stoffi was written in C#, you should find a toolkit that has bindings in C#.
Qt has C#/.NET bindings for all of its modules (https://en.wikipedia.org/wiki/Qt_%28framework%29#Bindings). There's also GTK# if you decide to use GTK. Many people in the previous comments have recommended wxWidgets, but they do not (at least officially) have C# bindings.
Hope this helps
VirtualBox is actually also a good example of the negative sides of relying on a cross platform toolkit: while it draws the widgets using native routines, the layout of the widget does not conform to the UI guidelines. Mac OS X specifies a certain distance that widgets are supposed to keep from window borders and other widgets. Qt or wxWidgets do not enforce these distances, nor do they enforce the default type faces or sizes.
By violating the layout guidelines, an application can draw using native widgets but still look alien to a Mac user.
Ok, there's been several comments mentioning Qt4 and WxWidgets (both of which I've used before but only very, very, very briefly in other projects). However, I need to know what the drawbacks are when compared to making one interface for each environment (KDE, Gnome, Unity, LXDE, etc). What do I gain by going that route instead of opting for a "one toolkit to rule them all"? What are the drawbacks on Qt4 vs WxWidgets?