People using apostrophes to make nouns plural. It look's ridiculou's to put apostrophe's after all your word's that end in esse's. Pro-tip: Never use apostrophes to make anything plural. Yes, there is a single exception regarding individual letters or numbers, but that doesn't come up frequently enough to warrant putting apostrophes after every plural just in case. Just don't do it.
Inconsistent capitalization is a minor pet peeve. I don't mind if you call it america or America, or north versus North, but pick one and stick with it. I know there are rules about when you should use capitals and when you shouldn't, but it bugs me more when people just arbitrarily use either without any consistency.
There, their, they're. To, too, two. Then, than. Should've, could've, would've (as opposed to the "should of" varieties). This is shit they teach 5-year-olds. It's really not that complicated. I can understand not remembering the quadratic formula or the periodic table, but not knowing how to communicate effectively in a language you claim to be fluent in is inexcusable.
Honestly, I don't understand spelling mistakes at all at this point. Most browsers offer a basic spell-check, and even if you're a lousy speller or you're dealing with a complicated word, you can Google that shit in a couple seconds. For example, I thought the word was spelled "mituration," but I wasn't sure, so I Googled it and realized it was "micturition." I learned something and I got to avoid setting myself up for looking foolish for using obscure vocabulary incorrectly.