The first man to walk in space. The Soviet (duh) pilot of Voskhod 2, he stepped out of the craft for 12 minutes and 9 seconds on March 18, 1965. He was tethered by a 15 foot piece of twine.

If memory serves, the spacecraft that is sent out to investigate the disappearance of the Discovery in 2010 was called the Leonov after this gentleman. Most folks don't know who Alexi is, but those who saw the movie may recall his name. I find this a little disconcerting, since this was an astounding milestone for humanity, let alone the old USSR.