Here are some FREE Terrain/Heightmap editors that I have gathered and tested, language denotes opensource licensing.
TerreSculptor HMES (Alpha, Windows)
Lithosphere (Python)
Scape Editor (Binaries, C++)
L3DT Standard Edition (Free, Windows)
Torque3D Terrain Editor (Windows/Linux, C++)
Updated/Suggestions:
IrrRPGBuilder (MIT/X11)
Artifex Terra 3D (Free, Windows)
cheers 
Hi,
There is also this one that I made with a friend (Andres Jesse Porfirio) with Irrlicht (Windows/Linux C++). It can save tiles in OBJ or Collada
http://irrrpgbuilder.sourceforge.net/ License is MIT/X11
Thanks for links, very useful editors.
Blender and Urho seem to handle Terrain at 2048+ for me.
Ant Landscape for Blender.
https://wiki.blender.org/index.php/Extensions:2.6/Py/Scripts/Add_Mesh/ANT_Landscape
A method to create and manipulate height maps by linking to a plane and rendering as greyscale:
http://blender.stackexchange.com/questions/9030/create-and-manipulate-height-map-save-as-greyscale-texture
Also, there are real world elevation data with with 30m resolution, avaliable for everyone. I was downloading it from here: gdex.cr.usgs.gov/gdex/ . It requires registration, and there may be other sites, where it can be downloaded.
en.wikipedia.org/wiki/Shuttle_R … hy_Mission
This is great guys and this thread is also meant for you to share your own findings and feedback.
@christianclavet
Yes, that is also one cool editor that you guys made with Irrlicht. Thanks for sharing!
cheers
Hello everyone,
you can always have a look at “Artifex Terra 3D”, also free (as in beer) and I am always very happy about feedback and critics.
I am currently in the process of reworking it and making it more agnostic and open, as in integrating assimp and plugins - specially export / import geared towards various engine formats.
edit I am also happily assisting with material conversion.
Cheers,
//Nauk
Hey Nauk,
Welcome to the forums! 
Yes, I got to try Artifex Terra when I was doing Ogre3D some time ago, and it really is a great terrain editor!
Here’s my built and a little test video 2 years ago :mrgreen:
I just thought it was more focused on Ogre3D so I did not include it on this list but it’s good to hear if you do have plans of making it agnostic/open and Urho friendly. 
Added now!
Nice Video Thanks for adding me!
//Nauk
Hi, all!
Please tell me which of terrain editors can be used with Urho3D terrain node? Or is there any reason in using it?
Sorry for probably stupid question - I never used terrain before, but tried it in Urho and don’t quite understand the workflow.
I tried using Terrain in Unity and see it as powerful tool, but is there something for Urho?
World Machine is the most powerful terrain tool, but it is a paid software, I have used it for several years.
Just thought I would chime in on this thread.
TerraSculptor is really awesome and really overlooked.
Wilbur. Its more of a height map editor than a terrain editor.
http://www.fracterra.com/wilbur.html
I took a look at that Wilbur tool. It has a few nifty little tricks. One in particular that is mentioned in the tutorials, basin filling, led me to a paper about implementing a basin filling algorithim. I decided to implement a similar technique in my own terrain editor based on that paper. For such a simple trick, it sure is handy for making terrains flow and drain more realistically.
One feature that isn’t very appreciated in terrain editors is the ability to stamp heightmaps. Having a good collection of height stamps gives freedom to level designers to create playable designs, without sacrificing “good” looking landscapes.
Also cool: this guys allows you to shade your terrains procedurally, but also exports a bitmap. So you can generate a heightmap, put through this, export a height, and then use in any other engine.
https://assetstore.unity.com/packages/tools/terrain/cts-complete-terrain-shader-91938