Scape is a non-commercial terrain editor I've developed as part of my thesis work in 2008. I've started writing about a few month back, and just posted a new article on its brush-based editing pipeline that's used to edit heightfields procedurally using brush strokes as input. Some of these brushes are implemented on the CPU, while others use the GPU to do the heavy lifiting.
As the series covers quite a lot of interesting technical graphics details and includes a number of working open-source procedural noise code snippets, I though I'd share it here as an hopefully useful source of inspiration and reference to others.
This looks really neat, I'll have to go over it later.
clb: At the end of 2012, the positions of jupiter, saturn, mercury, and deimos are aligned so as to cause a denormalized flush-to-zero bug when computing earth's gravitational force, slinging it to the sun.