I've been using L3DT to make maps for my OpenSim grids but it has a few shortcomings.

I've been using L3DT to make maps for my OpenSim grids but it has a few shortcomings. I uploaded the latest version of the Heightmap for the Twin Isles and in-world it looked like something I'd built in Minecraft! There's no easy way for me to create gentle rolling gradients manually - I have generate it from a design map or import an image. To create a gentle gradient manually in L3DT I have to use the "Bulldozer" tool. This is great for making roads and train lines, but looks horribly artificial. Otherwise I can attempt to smooth or avalanche things!

The editing features a very primitive. You can for example, only make a plain rectangular selection to copy and paste from, and you can't use that selection to do operations in either. :( So I've decided to upgrade to LEVELLER. It has a much better feature set and should be more fun to play with. I'll still be using L3DT, but mostly for general laypout and generation, and exporting to Opensim. Otherwise Leveller will fill in the gaps.
http://www.daylongraphics.com/products/leveller.php