IPlug is a simple-to-use C++ framework for developing cross platform audio plugins and targeting multiple plugin APIs. Originally developed by Schwa/Cockos, IPlug has been enhanced by various contributors. I have just published my version which now targets VST2, VST3, AudioUnit and RTAS APIs. It can also produce standalone Windows/OSX audio/midi apps and apps for Apple's IOS devices, all from the same code.
IPlug has a permissive licence and and comprehensive GUI functionality via Cockos' LICE (lightweight image compositing engine).
more info here:
http://forum.cockos.com/showthread.php?t=91811
Source code and example projects available on github
https://github.com/olilarkin/wdl-ol