I've found myself creating a lot of terrain for OpenSim lately, and without Windows and its fancy tools (read: Terragen) I built a workflow for making single or multi-region terrain maps with Blender and the Gimp.
It seems like this process might be useful to others, both with OpenSim and in Second Life. So I cooked up a tutorial about it:
http://stringofbits.net/2009/09/tutorial-creating-opensim-terrain-with-blender
If this tutorial is actually useful for you, or if you have any constructive criticism, please post! This tutorial is provided in the hope that it will be useful :)
If this is a bad forum for posting this, I apologize; building was the closest forum to terraforming I could find. :)