This is a recent fashion, supposedly made popular by "Posh" of Posh & Beck's fame. It can be anything from a simple feather on a band or comb, to a more elaborate concoction of flowers, tulle, sequins and of course feathers. It just adds the finishing touch to an outfit and of course also looks wonderful in a hat shop window display.
If you don't see what you want in the examples below, contact me to discuss your preferences.