Galileo is one of history's most famous scientists. His contributions to astronomy, physics and humanity's understanding of the universe are peerless. August marks the 400th anniversary of his eight-powered telescope that led to a twenty-powered telescope and his discovery that the surface of the moon is not, as had previously been thought, completely smooth.
In honor of his August telescopic achievements, Slate.com has a short article explaining why we refer to him by his first name only. Renaissance Italy was not a hotbed of name standardization, and surnames were fluid and inconsistently used. Galileo and Galilei were both surnames used in his family, making his name akin to "William Williams." The name Galileo, then as now, was unusual enough to stand on its own as an identifying mark.