Google routinely changes their logo to mark a point of history. Today it was the 50th anniversary of the Sputnik launch. The Sputnik ‘was the first artificial satellite to be put into geocentric orbit. Launched by the Soviet Union on October 4, 1957, it was the first satellite of the Sputnik program.’ [wikipedia].
If Google Doodles are your thing check out this page on Google
One of my favorites is Louis Braille’s birthday. They posted this one on January 4th 2006