I would like to issue a public thank you, gracias, humongous hug and many other accolades to Slate for their excellent article on why you should never, as much as the urge may strike you, use two spaces after a period.
For example, see the end of this sentence, right after the question mark? Single space.
As author Farhad Manjoo puts it, "the correct way to end a sentence is with a period followed by a single, proud, beautiful space." Typographers began to come to agreement on one space as a standard in the early 20th century.
However, the use of two spaces came into widespread use thanks to the typewriter. Early typewriters used monospaced type (for those of you into fonts, something like Courier or Monaco). Monospaced type, unlike proportional type, leaves more white space between characters and words, so the use of two spaces was adopted to make it easier to see the space between sentences.
Since monospaced fonts are rarely used anymore, with the vast majority of computer fonts using proportional type, there's no longer any reason for using two spaces. Typographers agree that it's just plain wrong. Generations taught to do so are still doing it, though.
High school journalism and reading teacher Samantha Jacobs told Slate that the reason she requires her students to use two spaces is that that's what she's used to. "Primarily, I base spacing on the way I learned." Slate quotes typographic consultant Ilene Strizver: "I talk about 'type crimes' often, and in terms of what you can do wrong, this one deserves life imprisonment. It's a pure sign of amateur typography."
I edit stories written by others for the website, which means I'm taking out double spacing at the end of sentences on a daily basis. In fact, the first thing I do when I open up a story to edit is do a find and replace for two spaces to do my small part in eliminating this affliction.
It's time we all take up the gauntlet and end this space plague. (I mean, it's not a plague from outer space, it's... you get the point.)
I'd also recommend this fine article on why Caps Lock keys are awful and outdated as well: GOOD RIDDANCE - The long-overdue movement to abandon Caps Lock