btgui
Tiny cross-platform GUI based on OpenGL 3.x core, gwen and stb truetype font for Windows, Linux and Mac OSX
Just create Visual Studio, XCode or Makefiles and you are ready to go.
Windows: build3/vs2010.bat
Linux build3/premake_linux64 gmake
Mac OSX build3/premake_osx xcode4 (or gmake)