Adopted in 1946, the flag of the United Nations has been used to indicate world unity, although it technically only represents the United Nations itself. It has a geographical representation of the planet, and its high visibility usage makes it a well-known contender for representing Earth.
Jolly Roger is the traditional English name for the flags flown to identify a pirate ship about to attack. "Jolly Roger" had been a generic term for a jovial, carefree man since at least the 17th century, and the existing term seems to have been applied to the skeleton or grinning skull in these flags by the early 18th century.
The Olympic flag represents all mankind and has a white bar with 5 interlocking rings of 5 colors. The five rings represent the five continents. The six colors (including the white color of the background) represent all of the world's nations.
The World Citizen is a social movement for global citizenship under a proposed world government. In 1953, one of its activists, Garry Davis, founded the World Service Authority, which issues the World Passport (considered a valid passport by only a few countries) with a proposed flag of the world.