Wire mesh fences are a part of modern life, and thus should be a part of modern battlefield terrain. Here’s a quick method for making fences from gutter guard.
modeling
Building A Wilderness Outpost
Terrainthralls has a nice tutorial on building a wilderness outpost which could be used in a variety of settings.
Fallen Trees and Bushes Stands
Here’s an article on making fallen trees and bushes terrain pieces that could be used to designate rough terrain in any game.
Making River Terrain
Here’s a short tutorial on making “wild river” terrain. The impetus for this was playing the Fords of Isen scenario, but the techniques obviously could be used for river terrain for any game.
Making Alien/Mutant Vegetation From Straws
The seminal site Terragenesis has an interesting bit on making alien vegetation from drinking straws.