Tea Tree oil is an essential oil made from Melaleuca alternifolia leaves.
It is known to treat dandruff, foot fungus and acne.
The average cost is around $12 for 1oz., so it's a little expensive but it should last a while.
I really love it for the soothing effect on the scalp. I mix it with conditioner I'm using for a pre-poo and do a quick scalp massage. I also use it for acne spot treatments. It dries the spot up fast, but be careful because it can really dry out the skin when used in high concentration.