I learned to type on a manual typewriter (circa 1986) and was taught two spaces after a period. I got a job creating training materials in the late 90s and my manager was an ardent lover of one space and made me correct every document. Now, I’m the training manager who’s a fan of one space after a period. When I get a draft written by someone else that will be used on our training site, I just do a quick find/replace double space to single space. No biggie.
Two. Never used typewriters much, but I learned to type on non-proportional WP software.
More impersonally, it depends on the style guide you’re using for your work. The Chicago Manual of Style specifically states there is no instance that more than one consecutive space should appear in an electronic work. If your style guide calls for two, then by all means.
Of course, this whole thread illustrates why editors exist: so writers can muck up the rules all they want and a decent document is still produced. 
The nice thing about this is if you run it multiple times, you’ll also catch the guys who sometimes put in 3 or more spaces for no apparent reason.
My answer to “Why do you care?” is that in certain non-ideal circumstances, two spaces has messed things up. In particular, writers who use two spaces (and other typewriter-style habits, like two hyphens for an em-dash) may use MS Word or some other word-processing program when writing, then send the work to an editor, who uses some other dang program and sends it to the copyeditor who uses a third program. Lather, rinse, & repeat for a few drafts. Then it’s sent to a compositor, who is using a completely different program, and who cares about what the thing looks like on the page, not what it says. A GOOD compositor watches out for any funky issues introduced by the warring word-processing programs, but you don’t always get good compositors. Even then it usually doesn’t matter, but if you toss in crap like tables or sidebars or what have you, and a particularly incompetent or overworked comp, it can get ugly.
In short, two spaces all on their own won’t make a bad ugly, but two spaces combined and split and stirred and shaken with all the other crap going on in a complex page can.
Outside a professional context, I just don’t like how two spaces looks. 
Two, like god intended it and thinking humans execute it.
I’m another professional editor chiming in to say it should be ONE space. Like others, I change this in many (most?) of the documents I edit, but it’s easier to just run the macro than to bug the authors.
I get paid by the word. Nobody’s paying me for all those extra spaces.
You also get those idiots who use the spacebar instead of the tab key.
I’m not an old fogey, but my dad is. He taught me to use two spaces when I was a little kid, and I’ve done it that way ever since.
When I learned to type in school – on a typewriter – they taught us two spaces. And even though I know it’s supposed to be one, it’s been hard to break the habit. I voted “one” because I know that’s the accepted style, but I often use two if I’m not concentrating on what I’m typing.
Two spaces was what I was taught back in ye olde typing classes in grade school, and that’s the standard I used up through my first job that involved proofreading or editing. Now I use one, because that’s the standard I’m used to. As for why one space is the standard at these companies instead of two, my personal theory is that it’s easier to ensure compliance with. It’s much faster to do a single find/change for two spaces to one than to cycle through all the possible combinations of sentence-terminal punctuation without accidentally getting an extra space in after something that isn’t sentence-terminal.
As with all standards, *consistency *is key.
I’m entertained by how close the poll has been every time I check in.
Less entertained by the fact that Two is always just slightly ahead. Damn you, typewriting class!
Well yeah, exactly. And I just can’t get that bothered by the two spaces that I would invest the time it would take to get used to it.
Fair enough. I just wanted to be clear that there wasn’t anything intrinsic to one space that would make it slower than two other than lack of practice.
Not all typesetting defaults to a single space. Any book typeset in LaTeX uses nearly two spaces (it’s a dedicated glyph for sentence endings somewhere between one and a half and two spaces), unless you tell it otherwise.
What? I was taught 5 spaces for indent. Why back in my day, the teacher’d up and wack my fingers for using tab to indent. I was learned to only use tab for aligning tables and such. Damn, and I didn’t think I was that old.
ETA: Oh, Tab was OK for outlines.
You had a bad teacher, or the software you were learning on used tabbing differently.
Ha! yeah… to both. It was on a DOS machine, white letters on blue background. Tab spaced over something like 8 characters.
She was in her 60’s and she gave us a speed test one time and we were supposed to type the same sentence over and over… I typed it once, hit SHIFT-HOME, CTRL-C, END, ENTER, CTRL-V, ENTER, CTRL-V, ENTER, etc. filled up my whole screen. She knew I cheated, but couldn’t figure out how.
I’m 23, and my Mama taught me two spaces, damnit! By the time they gave us our first keyboarding class I was entrenched in two spaces and never changed.
Edit: I also hunt and peck. My keyboarding exams were hilarious, because I’d come out near the top for accuracy and speed, and then get horrible marks for my form.
And this is why I’m against teaching religion to children.
not really