Oh, goodness, there are so many...
Several have been covered already, including your versus you're, irregardless, and "I could care less." In addition...
(Some of these are written form only.)
Using apostrophes for plurality. No. No. NO. NO! NOOOOOOOOOOO!
Affect versus effect.
Confusing i.e., i.e., that is, and e.g., i.e., for example, e.g., "Sometimes I like putting a condiment on my hot dog, i.e., ketchup or mustard." - which means that you are literally saying there are only two condiments in the world from which to choose: ketchup or mustard.
Peak versus pique. Nothing "peaks your interest."
Using "alot" - which is not a word - or "allot" - which is a word with an entirely different meaning - when you're trying to say "a lot."
Insure versus ensure.
Calling something ironic when it is simply just coincidental or unfortunate and not actually ironic.
Misusing literally, e.g., "That scared me so much I literally had a heart attack!" Um, no you didn't.
Using "should of" or "could of."
Double or triple negatives.
Hearing someone say or reading someone having written something like "Gosh, that literally shocked me to death when I learned it 'cause I was like there weren't no way I could of not never known that!" makes me literally want to shoot that person in the face.