Update

A new addition to an old favorite. MuggleNet has added 18 more items to their list of 115 Ways to Annoy, Harass, Confuse or Generally Scare Lord Voldemort. "Sure-fire ways to get yourself killed, or at least Crucio'd round the block and back again." Worth a read, or two, or three, or... well, worthy of memorization.

Mandrake

"The plant Mandragora officinarum, whose roots are used in magic and supernatural traditions. Despite being a popular ingredient for folk magic love potions in medieval European culture, the mandrake gained a sinister reputation that it still hasn't managed to shake off, due to the fact that its roots resemble the form of a human being. Mandrakes were commonly found under the bodies of hanged men at crossroads and they were said to thrive on the agony of death. According to lore, when mandrakes are harvested they shriek, leaving the harvester insane. All these negative associations are probably due to the high levels of psychotropic compounds found in the plant, which, if taken in high amounts, can cause delirious fantasies and visions."

-From "The Element Encyclopedia of the Psychic World"