Plugins

Breeze Designer Terragen Reader Plugin
A plugin for loading .ter files into Breeze Designer.
http://www.imagos.fl.net.au/breeze/plugin/terrain.htm

Camera Path Editor
An OpenGL spline-based animation plugin that generates script files from which you can create pictures to produce an animation in external program.
http://www2.cs.uh.edu/~somalley/campath.html

Firmament
A set of plugins with terrain import filters, terrain manipulation filters and images filters. Download only for windows.
http://www.mclusky.co.uk/firmament/

For Export Only
A collection of Terrain accessory plugins for Terragen that allow you to export terrains in a number of formats bmp, dxf, obj and rib. [Win 95/98/Me/MacOS]
http://homepages.ihug.co.nz/~jomeder/feo/index.html

WaterWorks
A collection of 7 water effects plugins that produce a transparent effect, ripples, waves, ice, lava, and surf like effect around the shoreline, and whitecaps. There is also a Mask generator, two tutorials and download area. [Windows/Mac]
http://homepages.ihug.co.nz/~jomeder/WaterWorks/index.html