It was a rude awakening when I finally learned that there should never be two spaces after a period. This was a few years ago and it felt like I was finding out for the first time that I was adopted. Two spaces as ultimate truth had been deeply ingrained through school. How much time had I wasted typing that extra space?

Quote from About.com:

It is generally accepted that the practice of putting two spaces at the end of a sentence is a carryover from the days of typewriters with monospaced typefaces. Two spaces, it was believed, made it easier to see where one sentence ended and the next began. Most typeset text, both before and after the typewriter, used a single space.