Using Leveller

Daylon Leveller
Dayton Leveller is a powerful 3D terrain editor for windows from Daylon Graphics. With Leveller the user can "paint" terrain, use vectors to create contours (which can then create heightfields) and use a variety of filters and tools to create distinctive and realistic terrain features (see the website for a more complete list of features). Users can also import and export heightfields in a variety of formats, and the latest update includes the ability to import and export tilesets of regions (in Terragen format) from/to Opensimulator.

Official Tutorials
Filling Holes

Harmonic Cycle
Import roads plug-in step by step

Leveller for OpenSimulator
Part 1 - Importing from Opensimulator into Leveller

Part 2 - Exporting from Leveller to Opensimulator

Part 3 - Creating Terrain using shapes in Leveller

Part 4 - Importing real life terrain (uses terrain.party for source files)

Part 5 - Leveller Documents (uses terrain.party for source files)