Quote Originally Posted by -=SoKrA=-
g++ is the default compiler for most UNIX OSs. You cold try gtk(+), Qt or wxWindows for the GUI. These toolkits are available in most if not all GNU/Linux distros.

Qt is free for Open Source apps, but gtk and wxWindows (or whatever it's called now) are under the LGPL which means you can link to them from a closed-source app.
Actually wxWidgets (changed from wxWindows due to threat from Microsoft I believe) has one minor difference in its license from lgpl, quoted below from their site
The wxWindows Licence is essentially the L-GPL (Library General Public Licence), with an exception stating that derived works in binary form may be distributed on the user's own terms. This is a solution that satisfies those who wish to produce GPL'ed software using wxWidgets, and also those producing proprietary software.