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.
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