This is a presentation about CityJSON and the variety of open source tools that are available around it. CityJSON is a file format that can be used to exchange 3D city models. The talk will focus mainly on ninja, the QGIS plugin and the Blender plugin and some details about their developments and our plans for them will be shown. A brief overview of all software options will be presented and the suitability to different applications will be investigated. Also, certain other state-of-the-art developments regarding CityJSON will be mentioned (e.g. dissemination through OGC API Features, serving data through 3D Tiles and versioning of CityJSON models).