Herbalsim

The Magic Leaf and other herbs.

Healing potions, poison antidotes and charms against evil. What is athelas or kingsfoil besides a flower? What effect can wolfsbane or garlic have? While being mostly mundane substances they can add a bit of magic and wonder to the game by applying a bit of imagination. Seemingly common remedies are too often overlooked in a milieu full of flashy magics. When used liberally, the herbal concoctions can make low-level magical effects more available and also add some unique flavoring to anyone's fantastic setting. They certainly do so in the famous works of J.R.R. Tolkien.

Smoke, tea, plaster and powder - the stuff of folk healers and hedge witches also makes for a lot of interesting game-play. The search for an herbal remedy, or a special spell component can form the basis of wilderness, underworld or social encounters. 

Appendix J found in the 1979 Dungeon Masters Guide is a good place to start when seeking inspiration for the inclusion of herbs in the RPG campaign. Other game resources I have found useful can be found in the Rolemaster (Classic) Character/Campaign Law book and in Maelstrom, a FRP game written by Alexander Scott and first published in 1984. Each of these games contains a section on using various herbs in a very "magical" way, adding to and enhancing game-play.

In his Riftwar Saga novel Silverthorn, author Raymond E. Feist makes use of both a potent poison and a plant based remedy or antidote as a plot mechanism. Author Katherine Kurtz makes use of a number of herbal substances and potions in her Deryni novels - some with unpredictable side-effects. In her novels, merasha has a debilitating, almost hallucinogenic effect on the magical Deryni folk, but has no effect on non-magic users. 

One doesn't have to look far to find many other authors who make use of various mundane or non-magical substances which can be prepared in various ways to produce effects which have wonderous power over the living or occasionally, the undead.