Prince William chose to call his brother Harry his best man at today's Royal Wedding. Traditionally, royal bridegrooms have had one or more 'supporters', not 'best men', as the official website of the British monarchy says (near the bottom of this page). This site also says that if the bride is royal but the bridegroom isn't, then the groom has a best man, not supporters.
The term 'best man' dates back to the beginning of the 19th century. It was originally a Scottish term.
If you haven't yet had enough of weddings, see this recent post of mine on the terms for other wedding attendants.