In the past several months, together with Kadaster, we have been busy developing 3dfier, an open-source software to generate LOD1 datasets from 2D and elevation datasets. The software outputs CityGML, IMGeo, and OBJ.
The render below shows Delft in 3D, reconstructed entirely from open data using our free software.
We have generated a set of sample data of a few cities in the Netherlands.
In future we plan to add new datasets, so feel free to bookmark the page and check it for updates.