One of the few available CityGML Viewers out there is LandXplorer CityGML Viewer from Autodesk.
As you can find out on their homepage, it is a proprietary product and at the moment only available for MS Windows Platforms.
This is normally bad news fro Linux users like me. But luckily there is wine.
I gave it a try, not really believing that it will actually run, but it does!

So, download it from Prof. Kolbes FTP (in this case you don't have to give Autodesk all your personal Information) and install it with wine.

If you do not have wine already running, the setup is very simple in Ubuntu, just do the folowing:

   sudo apt-get install wine

Open a terminal and browse to the folder where you have downloaded the LandXplorer Viewer .zip file into.

Unzip the File.

     unzip LandXplorerCityGMLViewer2009Setup.zip 

Run the Setup Program with wine
 

    wine LandXplorerCityGMLViewer2009Setup.exe 

Now you will see:

In my case it runs very fine without a problem. :-)

Loading a CityGML Model into the viewer:

This is what the GUI looks like with the example Model from Berlin Alexanderplatz:

best regards,
Christian