Thanks to @TextExpander and @miraz (via twitter)! Do you like the way iOS inserts a period and a space when you double space at the end of a sentence? Get OSX to do the same using TextExpander!

Create a new TextExpander group > set 'expand after:' Any character > create new snippet [space] [.] (that's a space and a period) > the abbreviation [space][space] (hit the space bar twice). Now when you double space at the end of a sentence you should get a [.][space].