Zircon Galaxy is a project created with free resources, OpenGameArt was major resource used for this.  It is open source.

Quaternius vehicle models come to mind as one of the most vitial resources, I had a very hard time locating vehicles with a reasonable vertex count that also renderered nicely -- but there were many models and sounds here that proved invaluable.

If interested, a moddb link:

https://www.moddb.com/mods/zircon-galaxy-alpha-10