CinsImp is a clone of the famous classic Mac application - HyperCard. HyperCard was revolutionary in it's time and probably the inspiration for the world-wide-web (what most people today call the 'Internet').
There have been various implementations of CinsImp over the years. One was fairly complete, the others I had done substantial work on. At this time, unfortunately, none are under active development.
For now, you can find various things CinsImp-related collected on this page. And yes, CinsImp is open-source.