fOR REAL:   It depends on the context.  In journalism, it's one space because the paper used by those extra spaces costs.  In final publishing, the extra space is usually removed (check your paperbacks, you'll see).  However, MSS are still turned in to publishers with two spaces, and APA 6 still recommends that all drafts use two spaces.

 

This originated with mono-space typewriter fonts; the unevenness of the kerning made it easier to identify the end of the sentence if there were two spaces there.  Modern fonts don't require this; the extra space is still useful for clearly marking the sentences if your eye is trained that way.  Mine eyes are trained that way; when I'm proofing, the missing space is arresting. 

 

Dragged from the recesses of my drug-befuddled brain--

 

Anna Holloway