CityGMLViewer is, as the name implies, a CityGML viewer. It is based on OpenGL, and was developed by Matthias Betz at HfT Stuttgart.
It works on Windows and Linux.
It can display CityGML 2 and 3, and can show larger files than FZKViewer.
Here's the most recent version (v0.0.1).
start.bat script inside the zip file, which can be executed on Windows.
As an alternative, you can also download the code and compile it yourself:
git clone https://transfer.hft-stuttgart.de/gitlab/citygml/citygmlviewer.git cd citygmlviewer mvn package