A few that really get under my skin:
But more than any of this, I think my biggest grammar/usage/spelling pet peeve is when someone pounces on someone else's mistake and makes that the focus of the conversation. Especially in a Facebook / message board context, which I've always felt is more conversational anyway and therefore more prone to errors. (Everyone make usage errors in casual conversation, after all. That's the nature of spoken language.)
2. Punctuation with quotation marks and parentheses. (Periods and commas go inside quotation marks. But people get it so consistently wrong, it's probably just a matter of time before the wrong way becomes standard.)
Do you know what drives me crazy? Nearly every published novel I've ever read has at least one obvious typo. Who edits this shit? I mean seriously if I can catch it while reading the book, why didn't anybody catch it before it went to print? Why didn't they correct it before it went to paperback? It almost makes me want to edit the error and send it back to the publisher just to say, "You suck at your job!"
ditto to all who said adding an apostrophe to the plural.
But, I hate, hate, hate how "myself" has worked its way into the vernacular, and has become interchangeable with "I" or "me". Ex: Jim and myself went out to dinner. Grrrrrr.