Towel racks can be made from almost any material imaginable. Metals are common, as they can look extremely attractive and will not be affected by consistent dampness in your bathroom. For a more natural look, you can consider wood as well; however, know that if you choose a wooden towel rack, you'll need to make sure it's water-treated so it can withstand inherent moisture.
For outdoor showers, consider a plastic or vinyl version. These will stand up to the weather quite well. The best luxe option is likely high-density copper: It will not wear or rust, and it will look beautiful for years to come.