Cross Platform GUI

[Alpha Omega]

I am looking for a free to use library that offers cross platform Graphic User Interface Support. I want to make something similar to using Windows Forms for .NET. Anyone have any good ones?

[fmx]

Qt?
http://qt.nokia.com/

[Radikalizm]

Qt?
http://qt.nokia.com/

I'm going to advise Qt as well, doesn't work as intuitively as WinForms and needs some getting used to, but it's a good alternative

[eye776]

Lazarus works ok and it's very much cross platform if you're cool with using your C++ application as a .so or .dll.( That and you don't mind writing your front-end in pascal ).
Well, at least that's what I use for my latest pet irrilcht project (it's some kind of animation/skeleton/material hybrid editor).

Other than that Qt or FLTK are OK.
FLTK might be interesting if you're used to irrlicht's GUI, IMHO they work in a pretty similar way

[Virion]

Qt is pretty huge in size (depends on how many component you included in your project). but it has a pretty neat editor which is definitely a plus for me. won't use it for in-game though only for tools-making such as level editor.