Surprisingly enough, some essential oils do have research-based actual uses, such as topical antibacterials, antifungals and antiparasitics.

While there's quite a bit of woo woo around them, there's also a lot of interesting research into phytochemicals like essential oils. Same with a few other "plant-based" things like pine resin; there's even a clinically tested pine resin salve that helps with wound healing and is used for treating difficult wounds in some hospitals in Finland.

The problem with essential oils is trying to filter out the snake oil claims from the actual research-based claims. Most vendors tend to have pretty, well, wild claims about what their products can do, so your best bet is scholar.google.com or www.ncbi.nlm.nih.gov and the like