Rosemary (Rosmarinus officinalis) has a long history of use from the time of the Greeks and Romans, with appearances in royal gardens, poems, and as a study aid. Rosemary essential oil is a great addition to an office or dorm diffuser, or anywhere a mental boost is needed. This marvelous mental stimulant is said to encourage alertness, and improve memory. It can also be diluted and used topically to massage sore muscles.