Chartreuse

Chartreuse is a very old liqueur created by monks. It comes in both green and yellow and is a staple in many cocktails.

From Wikipedia:
Chartreuse is a French liqueur available in green and yellow versions that differ in taste and alcohol content. The liqueur has been made by the Carthusian Monks since 1737 according to the instructions set out in a manuscript given to them by François Annibal d'Estrées in 1605. It was named after the monks' Grande Chartreuse monastery, located in the Chartreuse Mountains in the general region of Grenoble in France. The liqueur is produced in their distillery in the nearby town of Voiron (Isère). It is composed of distilled alcohol aged with 130 herbs, plants and flowers. It is one of the handful of liqueurs that continue to age and improve in the bottle.