When I learned how to type, back in the Stone Age, I was taught that you always used two spaces after the period at the end of each sentence. Consequently, that's burned into my brain. Apparently now, however, the rule has changed ... and only one space is required (in fact, preferred!).
This doesn't appear to be just one person's opinion ... Grammar Girl cites the Chicago Manual of Style, the AP Stylebook, and the MLA as all preferring one space after a period.
This is somewhat stunning to me ... of course, most of my typing these days is done in markup languages like LaTeX or HTML, so my perpetual use of two-spaces after periods would have no effect on how they're displayed. Still, it's curious.
Ok, all you young folk out there ... when you learned to type, what rule did you learn?