When i first needed a terrain editor for a 2D game i though that it was something that didn't existed, or that only game development companies had for their own games. After some time googling i found a lot of terrain editors. Some good, some bad, some cheaper, some more expensive. I thought for a while and then i bought T.E.D. I like this terrain editor, but didn't outperformed my expectations. My real intention was to use it with the Dark Basic game engine, but seems no one could help me doing that on the Dark Basic forums and there is no documentation for that matter (at least by the time i was trying to do it). But, besides that its a very good terrain editor. I still use it today and its my favorit for game development, even when used in other programming languages, like actionscript. I use it even for 2D games.this last semester at college i learned to use another terrain editor: TERRAGENIt has a lot of features that can be used for a lot of things, including for video previews for games. Its very easy to control the clouds, watter, sun luminosity, to raise and lower the land,... and that camera navegation, is "brutal", absolutly amazing. You can make move on the terrain with a camera view, controling the camera as you wish and need. The version i used wasn't very useful for 3d rendering but for 2d it was great. Because it was a very old version, i guess the new ones might be a lot better on creating 3d. I recommend it for sure.
Here are more previews of the 2d