Dynamo [1] has a similar feature set and is technically open source but it won't work without Autodesk's geometry engine and I presume it's mostly Autodesk engineers contributing to it. Useful if you want to do procedural modeling in Revit though Grasshopper also supports Revit.
Not open source but Nodepen [2] is kind of like Grasshopper but web based.
[1] https://dynamobim.org/
[2] https://nodepen.io
