wxWindows

wxWindows is a free C++ GUI framework and has a historical relationship with the Microsoft Foundation/Frustration Classes GUI framework.

features

• Platform native widgets. Occasionally additional widget code has been added to the wxWindows library to support a control (that is not available on a certain platform).
• No themes; this is inherent to the platform native widgets.
• Visual development environment (wxDesigner).
• OpenGL integration.

platforms

• MOTIF
• Windows
• GTK
• Mac
  

pricing

That's simple, it is free and available under L-GPL (Library/Lesser General Public Licence), with an exception stating that derived works in binary form may be distributed on the user's own terms.