Free Raylectron SketchyMesh plugin.
Create terrain and shapes from a grid using formulas in the form of z=f(x,y), very easy to use.
SketchyMesh is an easy terrain and shape builder. It is based on the formula z=f(x,y) for a grid where you enter your own ruby formula using x and y. You can download it from http://Raylectron.com it is free with source code but is (c)2012 SoftByte Labs Inc.
You don't need to know math, really, just modify the ones that comes with the plugin and see what it does. There is an Undo button to easily undo the mesh, so you can change the formula quickly on every turn. You can go the "View" menu and turn on "Hidden Geometry". My examples above have a material to them, but the plugin doesn't do that, it only create a mesh.
All in Ruby with source code.
•After installation, you will find the Raylectron plugins in the Sketchup 'Tools' menu.