To commemorate the passing of Kyser Capos founder Milton Kyser, The Unique Guitar Blog is running an in depth post about capos. The article covers the history, usage and variations of the device. For example, were you aware that classical and flamenco guitarists also capos? From UGB:
Capos are still being produced for classic and flamenco players. These devices, known as cejillas, are made of an ornate piece of wood or ivoroid that usually has a strip of felt on the bottom.
Read all about it and see photos at UGB.