wxFormBuilder is a RAD designer for wxWidgets. It is free and open-source. It is in the beta phase of version 3.0 and is also supplied as part of wxPack.
wxFormBuilder is easy to use, simple to look at, and provides a very decent set of features. It is a standalone system that is simultaneously good and bad, goosd that it requires nothing else, bad because some people require IDE integration. You can output the designs to XRC (the XML based resource language for wxWidgets) or to native C++.
You can see some screenshots of version 3.0 here. The screenshots so some fairly interesting and complex designs demonstrating some of the power of the system.
You can see their roadmap for features here. They use flyspray for their bugtracker (which is a good system).
What is particularly nice about this program is the amount of GUI widgets it supports including the contributed ones like wxFlatNoteBook. I thin it supports enough to create a very good GUI visually.