A potion to cheat death... almost.
Argaiv Silaic, a wise, excentrical explorer and physician, once discovered the blue mushroom's secret, and had a chance to note what he found out.
Faerie foods with unusual effects. Like faerie sausages that turn the eaters into wiener dogs, faerie wines that make the drinker whiny, etc. There's a table in Palladium Fantasy RPG which has a table of ideas.