Terraform is an open source interactive height field generation and
manipulation program, giving you the ability to generate random terrain and
transform it. It uses the GNOME desktop platform and thus has a (more or less)
consistent graphical user interface which doesn't require use of the command
line.

Terraform allows you to generate random terrain using a number of algorithms
and then selectiveley change the terrain using a variety of transformations.
Where possible, the transformations provide a real-time preview, giving you
instant feedback on the effect of any parameter changes.

WWW: http://terraform.sourceforge.net/