Worldsdesigner
You have the Abillity to place structures exactly where you want or let the computer handle all the descision.
This gives you complete creative control and still enables a fast workflow for giant worlds.
When letting the computer take over you can still control how the height of the landscape is respected.
Structures can be placed right on the floor, flying in the air or underground like caves.
The Software also automatically places structures non-overlapping except if you explicitly want that.
This is what allows you to place entire forests in a matter of seconds.