Concerto is a use-case designer with simple analytics for cost estimation built in. It's built on the Qt Framework (version 5, but 4.8 does it too ;) ).

(More content to come when I have the time to write more!)

This software requires Boost's Graph Library (BGL) installed in order to be built. If you're using qt-creator to build this using the *.pro file provided, then make sure that your compiler can find boost's header files in its search path.