They're/Their/There is another set of words that are commonly misused. They're means "they are," their indicates possession, and there refers to a place. I like to remember, "They're going to their house over there."
Then and Than are a couple more tricky words that are often confused. Than is a conjunction introducing a dependent clause of comparison. Then is an adverb meaning "at that time" or "next." For example, My car is faster than yours." and "We ate and then went for a walk." A "quick and dirty" tip from Grammar Girl: Both "than" and "comparison" have the letter "a" in them, and "then" and "time" both have the letter "e."
And finally, your versus you're. My guess is these two words are often confused for the same reasons as affect/effect and then/than. They sound exactly the same. Your is the possessive form of you, while you're is a contraction of the two words you are. "You're going to be happy I answered your questions."
When I was trying to decide which words to cover in this post, I quickly learned I could write a book about all of them. Which commonly misused words top your pet peeve list or which ones do you often struggle with? Comment below!