The Planetary Society
The Planetary Society, founded by Carl Sagan, bills itself as "the world's largest space-interest group". The Planetary Society's primary interests are science missions, strongly favoring robotic missions to the planets of our solar system. They have run a number of major projects over the years, including a so-far-unsuccessful attempt to launch a solar sail, Cosmos-1.
The executive director of the Planetary Society is Louis Friedman, and its current president is Neil deGrasse Tyson.