Seeking open source software similiar to Esri City Engine?

You could look at Blender which is the defacto standard open source 3D modeling program with a huge community and tons of tutorials out on the web.

Here is a tuturial about creating a cityscape in Blender.

3D City DB may be an option for you.

It is "open source and released under the terms of LGPL" and supports CityGML, Oracle and PostGIS.