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.

Firmament
A set of plugins with terrain import filters, terrain manipulation filters and images filters. Download only for windows.

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]

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]