The 3D City Database (aka 3DCityDB) is a free and open-source geo-database to store, represent, and manage virtual 3D city models on top of a standard spatial relational database. The database schema implements the international CityGML standard with semantically rich and multi-scale urban objects facilitating complex analysis tasks, far beyond visualization.
The 3D City Database 4.x comes with a set of tools, altogether grouped into the "3DCityDB Suite", which allows, among the rest, to export the database contents directly as KML, COLLADA, and glTF formats for the visualisation in a broad range of applications (e.g. Google Earth, ArcGIS, and the WebGL-based Cesium Virtual Globe).
Please note: The 3DCityDB (for PostgreSQL) is required by the 3DCityDB Tools plug-in!