IPlug is a simple-to-use C++ framework for developing cross platform audio plug-ins and targeting multiple plug-in APIs from the same code. Originally developed by Schwa, IPlug has been enhanced by various contributors. Oli Larkin has recently published his 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.
IPlug has a permissive licence, and does not cost anything to use in closed source projects.
IPlug depends on WDL, and that is why this project is called WDL-OL, although most of the differences from Cockos' WDL are to do with IPlug.
More info: cockos.com/showthread.php?t=91811
Source code and example projects available on github: github.com/olilarkin/wdl-ol