Ramen was an open source node based compositor (probably the first open source).
It was developed between 2008 and 2011, in C++ using the Qt frameworks and other libraries commonly used in VFX like boost, Intel TBB, OpenEXR, OpenColorIO, ...
It had many advanced features usually only found in commercial compositors.
In addition, it was a good platform for experimentation, for example writing custom nodes.
It was used in some of my compositing projects.