Jimbibbly is the man for a full answer. My favourite, although a little thick, is now OOP and was resin from TM Terrain?
The best substitute I have found, and what I currently use is textured plasticard from Plastruct (and its a bitch to get hold of in the UK without paying ott)
I have used ribbed rubber carmats, bamboo skewers, split drinking straws, you name it over the years