Tea Tree Oil
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!