This article in Slate looks at the most important issue of our time: Annoyance at people who insist on putting two spaces after every period.
Two-spacers are everywhere, their ugly error crossing every social boundary of class, education, and taste. You'd expect, for instance, that anyone savvy enough to read Slate would know the proper rules of typing, but you'd be wrong; every third e-mail I get from readers includes the two-space error. (In editing letters for "Dear Farhad," my occasional tech-advice column, I've removed enough extra spaces to fill my forthcoming volume of melancholy epic poetry, The Emptiness Within.) The public relations profession is similarly ignorant; I've received press releases and correspondence from the biggest companies in the world that are riddled with extra spaces. Some of my best friends are irredeemable two spacers, too, and even my wife has been known to use an unnecessary extra space every now and then (though she points out that she does so only when writing to other two-spacers, just to make them happy).
When I learned how to type (on an electric typewriter—what up, Maine public school system?) I was a two-spacer, but as soon as I started writing for money, I learned to use one space at editorial request. It wasn't that hard to do. Two spaces made sense for clarity in typewritten letters, but I think they're unnecessary now. Still, as Edward Champion points out, writing a 1500-word essay on this is a pretty douchey thing to do.