Tea Tree oil is a great addition to your preps. The plant is native to Australia and is related to eucalyptus. It has been used for centuries as an antibacterial, anti-fungal and infection healing oil. This is the tea tree oil I buy (tea tree oil)
It can be used on almost any skin ailment including blemishes, wounds, dandruff, head lice, chicken pox and blisters. It is fairly effective at killing Candida, which causes yeast infections.
It has similar healing properties to lavender oil (which I wrote about here).
You can grow your own tea tree plant indoors in containers and potentially distill your own essential oils from it but it requires a significant amount of the plant to make a small amount of oil.
It should not be ingested without supervision of a doctor as it can be toxic internally. Some people are sensitive to tea tree, so make sure you check for allergies before using. A simple skin allergy test is to put a drop or two on the inside of the elbow and wait for at least 24 hours to see if there is a reaction (ie: rash, hives or blisters). If there is no reaction, it may be safe to use it in greater quantities. If there IS a reaction, discontinue use.
Some essential oils are not safe for children, animals or pregnant/nursing women. Do your research!
Peppermint is a great herb that is easy to grow in most climates although it does best in shade with moist soil.
Homesteaders and preppers who keep bees are fond of this plant because it produces a lot of nectar that will attract the bees to the plant. For this reason, it is advised to grow peppermint away from the entrances of your home. It spreads easily and quickly so it is best to grow it in a container.
Dried peppermint leaves make an excellent tea that can help calm upset stomach, aid digestion and can treat flatulence. The essential oil of the peppermint plant can be used as a pest repellent. Mice, ants and squirrels do not like the scent and it can be sprinkled around your food storage or entrance points of the pests. For people, the scent can be used for migraines, stimulate the senses, and it has a cooling effect on the skin. It can be used for nausea and motion sickness but the oil should be avoided during pregnancy.
Make sure to buy peppermint plants as there as no seeds! They grow easily from cuttings if you have a friend with some.