"Artisan" is an amazing and incredibly useful organic modeling toolset released by SketchUp plugin wizard Dale Martens (a.k.a. Whaat). Based on Subdivide & Smooth tools, Artisan is perfect for people who want to use SketchUp to model organic shapes and terrain features. Artisan includes a set of "deformation" tools that allow you to sculpt, smooth, flatten, pinch and apply textures just like you would with a brush.
You can add or reduce polygon complexity in your model, allowing for more or less detail. There's also a suite of vertex tools that you can use to model based on controlling vertex points.
For more information and video tutorials on the Artisan Organic Toolset for SketchUp, check out this website.