Three of the most commonly confused words are their, there, and they're. However, they all mean very different things.
Two more words are its and it's. One shows ownership, and the other is used to mean "it is"
Then there are who's and whose, which are also used as ways to describe someone or show ownership, respectively.
Then there's good and well which, while having similar meanings, are different parts of speech altogether. One is typically used to describe a person, thing, or feeling, while the other describes the quality of an action.
Then there are affect and effect, which are meant to be used in different tenses. One describes how an action will influence future actions, and the other describes how an action or condition has been influenced by those in the past.
Finally there are accept and except, which are completely different. One signifies receiving a gift or offer, and the other means to exclude someone or something.