Stop putting two spaces after your sentences, you look silly / by Jamie Young

I suppose this will forever be debated...but double spacing is an archaic practice for typewriter use. Forget what you learned years ago. They were wrong. This practice should not continue. 

The two space "rule" was used back when people used typewriters. The issue here was that the typewriter characters were not each equally spaced: some were wider than others, while some were very skinny and took up hardly any space at all. Essentially, it wasn't pretty; the typing looked very uneven. To remedy this awkwardness, and to allow people to read what was typed better, people began adding two spaces after each period so there was a more prominent end. 

Enter modern day: It's 2014, people. We use fonts that are precisely designed to be proportional — and have been using them for nearly 30 years — so we don't have to do anything silly like put two spaces after each sentence. 

From Wikipedia on sentence spacing:

The Complete Manual on Typography (2003) states that "The typewriter tradition of separating sentences with two word spaces after a period has no place in typesetting" and the single space is "standard typographic practice".[36]The Elements of Typographic Style (2004) advocates a single space between sentences, noting that "your typing as well as your typesetting will benefit from unlearning this quaint [double spacing] Victorian habit."

Another great response to the question from Writer's Digest, here

Putting two spaces after a sentence now is silly. I don't even know a better word to describe it. That's it. Sure, back when you typed on a typewriter with an uneven typeface it may have made it easier on the eyes — but now? — now, it makes it harder on the eyes. 

You're drunk, double spacers. Go home.