Every other thing I do at home includes essential oils. They might seem pricey, but they're so concentrated and so useful, that they're 100% worth it, especially if a local co-op is willing to let you order direct from their distributor with them. Housing co-ops (include the one at your local small liberal arts college) and buying clubs are great for this. If there aren't any in your area, ask your food co-op/health food store, though they need to make money somehow, and since you won't be buying oils in bulk, you might have to pay full price.
Essential Oils are highly concentrated medicinals. Keep them in a cool, dark place away from children. Never drink or eat them (unless specifically indicated). This is the distilled essence of the plant. Treat them with the due respect they deserve.
Here's my stash and what I use it for, in order of how much I love it. You can find more info just about anywhere.
Lavender - It's as close as you're going to get to all-purpose. Smells great, calming and uplifting, Lavender is also a great antiseptic. use it on cuts and scrapes to ward off infection, in household cleaners, and in scented anything. I always add lavender to my baths, I run it through my hair to freshen up (just a drop or two), and if anything needs an uplifting scent, I spray it with a bit of lavender water (essential oil mix).
Tea Tree Oil - go ahead and buy the 2 oz bottle. Tea Tree Oil is an outstanding antifungal. It works much better than almost anything else (including many over-the-counter creams). If you have any type of fungal infection including athlete's foot, a yeast infection, or those spreading pink and brown spots anywhere on your skin, keep a mixture of tea tree oil and witch hazel on hand (20-30 drops tea tree oil in a half pint jar filled 3/4 of the way with witch hazel is perfect). Shake the mixture and dip a cotton ball in, squeeze out excess, and apply to the affected area twice a day until infection clears and then at least an additional week. I also include tea tree in all of my household and personal cleaning mixes. I mix 10-15 drops into my deodorant (1 part baking soda and 1 part corn strach - mix thoroughly after every drop. I keep mine in a tightly sealed mason jar and apply with a large powder brush. This is also great for whisking away moisture on feet, in cleavage, etc.)
Lemon - I use lemon essential oil in my dish pans as I do dishes. My water smells very strongly of sulfur, and washing dishes in batches in dish tubs with 4-5 drops of lemon does the trick. It smells so fresh that I can almost convince myself that I enjoy doing dishes.
Sage - Like its fresh and dried counterpart, sage is great for coughs and any sort of congestion. I add sage to my baths if I'm feeling a cold coming on. It is also part of my personal bouquet of scents. it is one of my favorite smells - being earthy and flowery at the same time, and also evoking that edge in the smell of body odor that is so mysteriously sexual, intimate, and calming (at least to my nostrils). I mix 10-15 drops into my deodorant along with tea tree oil (see tea tree section for recipe).
Sandalwood - I just love it. It's a scent thing. Also, all of your musky, wood smells are great in foot baths or regular baths after a hard day of work. They do something to really get in and relax your muscles and your mind. Cedar, Bergamont, Egyptian musk are also great "masculine" scents if you want to mix a scent.
Clove - clove oil is a strong antifungal and antibiotic. Use with tea tree oil if your infection really isn't going well. Also drink clove tea for general systemic candida overgrowth (but not essential oil - boil whole cloves).
Eucalyptus - Eucalyptus is most commonly used for clearing colds and minor congestion. Carry a small vial with you and inhale the scent as needed to help with a common cold.
Echinacea - Mix 10-15 drops into water or juice and drink to boost immune system response.
Oregano - Mix 5-6 drops in water and drink to ward off cold and flu-like symptoms, and to generally boost immune system response.
Garlic oil - This isn't an essential oil, but it is a great medicinal oil. heat up 1-2 tabelspoons of olive oil in a small pan as if to cook. add a clove of minced garlic and turn off the heat. let it sit in the pan until the oil is warm to the touch. strain out garlic and massage the oil into ears to treat ear infections. Lay on each side after applying to each ear to let it seep in.
A note on mixing your own essential oil perfumes:
No one appreciates being bashed over the head with your scent, no matter how good it is. go for subtle. Too much Egyptian Musk makes you smell like a bad male escort. Too much rose makes you smell like an South Asian kitchen before a wedding. Mix small amounts and then add to it, so you don't waste excess on a bad mixture. use two drops, max, on the throat, behind your ears, wrists, and temples. never perfume your cleavage. it's trashy.
Check in with this entry from time to time. I'll be adding more as I think of them.