In the course of a technical session at JavaOne, Joshua Slack and Rikard Herlitz showed off the kind of tools you can build using the jMonkeyEngine, and used the NCSoft world-building tool as an example. This video shows Rikard putting together a hasty landscape that comes out quite nicely, given that he is 'just a programmer' rather than a professional level designer. (Self-deprecatory humor for the proverbial win.)
Although the video is intended to demonstrate the capabilities of the jMonkeyEngine, it's an interesting look at one of the tools used by a major player in the MMO market. NCSoft has been using jMonkeyEngine for the last two years. Joshua doesn't mention any specific games that have used the tool, though he does hint rather strongly that NCsoft's most recent output might have employed it.
The worldbuilding tool presentation follows behind the break.