C++ UI resources

wxWidgets is a cross platform GUI library for C++ (and other languages). The main site should have enough pointers to resources to get going.

You might also want to check out this question/answer here on stack overflow if you are specifically thinking of Windows

If cross platform support is important then I would second the suggestion to look at Qt. It supports Windows, Linux and the Mac. For free software it is free (there is a GPL version on Unix but not for Windows) but for comercial software it is not particulary cheap. There are now several books on Programming with Qt.

It does come with a large number of extra libraries for networking, parsing XML etc. It also has integration with Visual Studio on Windows.

One downside with Qt is that there are not as many add on libraries as with some other GUI frameworks. Ot will depend on the type of applications that you wish to write whether this is important to you or not.