Melaleuca alternifolia (tea tree oil)- A rather long article, with attention to research on different uses, is found at the Mayo Clinic website. Uses, based on scientific evidence, are all class 'C', or "Unclear scientific evidence." In other words, the research results are inconclusive.
Note that it is suspected in some cases of contact dermatitis. Note that there are other species and genera, and the article applies only to M. alternifolia.