I have only use Qt (www.trolltech.com) a bit, but it's 
a very clean and easy to use toolkit, available for 
free for Windows and X11 (if your code is GPL'd).
I have also heard good things about wxWindows, fltk 
and FOX.
Check out <http://www.free-soft.org/guitool> for a 
comparison of dozens of toolkits.