You can find metal as well as plastic or rubber feet.  Bag feet using prongs and washers (left on the picture) are most common in the DIY-sphere as they are very cheap and easy to install requiring no special tool. They tend to be prone to loosing their cap over time however. Bag feet that are attached with a rivet (not pictured) or screw (pictured on the right) are also widely spread nowadays. They are more expensive but this pays off in terms of longevity.