Darwin and Mendel discovered key roles of variation in biology. Some useful tools for quantifying, understanding, and exploiting variation are not sufficiently widely known among biologists. Taylor's power law of fluctuation scaling describes a relationship between the variance and the mean of a positive quantity. Examples of Taylor's law include the population density of bacteria, rice, wheat, potatoes, trees, triatomine bugs that transmit Chagas disease, fish, rodents, and humans; the numbers of cells per mammalian organ, parasites per host, cancer metastases, and single nucleotide polymorphisms; and non-biological quantities such as the prime numbers, the number of tornadoes per outbreak, and the volumes of currency exchanges and stock trades. Taylor's law results from a wide variety of processes, so inferences based on Taylor's law require care. This talk will be partly a tutorial addressed to the question: What can Taylor's law do for you?