Hemlock, made famous by the execution of Socrates, was no relation to the conifer. It comes from a plant in the carrot family found in Europe and North Africa. Every part—leaves, seeds, roots—is toxic when ingested. The toxicity remains up to three years after the plant is dead and dried, making it easy to import from its native land. It was less popular in the Middle Ages than in Classical Greece.
Aconite (monkshood, wolf's-bane, blue rocket, et alia) is common all over the Northern Hemisphere. It contains an alkaloid toxin called aconitine, which can "lead to diarrhea, convulsions, ventricular arrhythmia and death." A sufficient dose causes death within two to six hours.
Datura (jimsonweed, thornapples, devil's trumpets, moonflower) is extremely poisonous but was used by many North American tribes for its psychoactive properties. It is found all over the world (the illustration is the Western Hemisphere's datura inoxia), and Indian Thuggee practitioners used it routinely on victims in their sacrifices to the Hindu goddess Kali.
Henbane was used as an anesthetic, especially when combined with mandrake, deadly nightshade, and datura. Pliny said its use was similar to taking wine and therefore "offensive to the understanding."
Mandrake has long been used for magical rituals because of the branching root that vaguely resembles a human body, and because of the hallucinogenic alkaloids. Many in the 21st century likely heard of it for the first time because of the Harry Potter books and films.
Of course, these substances were available from any apothecary, or were cultivated and prepared by an individual without raising suspicion, because in small doses they were medicinal. Hemlock was used as a sedative and for swollen joints. Aconite in very small doses was thought to improve circulation. Dioscorides recommended henbane as a sedative.
I have deliberately skipped over arsenic, because it has such a long and glorious history that I felt it deserved its own entry, so please join me tomorrow for that.