If it wasn't obvious from previous post - I too recommend Qt.
It uses the native API in each OS, so on windows you can't distinguish it from something programmed in win32API, on mac it uses aqua, on linux and BSD it has a kde look...
apart from the easy meta object way of doing things in Qt (connecting objects with signals to slots makes event handling and threading very easy),
Qt also provides integration with openGL for 3d rendering inside a window,
string manipulation with regular expressions, Unicode and easy conversions between language codecs,
better containers then STL has to offer (list, vector, map etc.),
TCP\UDP client and server base classes, ftp and http clients, the labels and other visual text containers parse html, and in the next version (4.4) there will be some classes which handle very sophisticated sites with code from webkit (mac's safari and KDE's konqueror web engine ).
oh and they will also have sound infrastructure using code from phonon (KDE's sound library) in 4.4...
MFC is really ugly... it's more C then C++, and it's very easy to get lost in your code when using it... I recommend you avoid it.