The best simulators I have ever stumbled upon. Rigs of Rods is free and BeamNG is $20 with very slightly better graphics and physics. I have both, and they're awesome. They are designed for modding and creating content. I think it is easier to make a vehicle in Rigs of Rods, since there is a tool for that, and BeamNG makes it easy to create terrains (it includes a terrain editor).
Discuss it and post your screenshots here. Here are some pictures from BeamNG:
Some of RoR:
