Saylor.org's English Composition/Frequently Confused Words and Spellings
No matter how strong the argument or style of writing, spelling mistakes are distracting and reflect poorly on the author (both as a writer and editor). Computer word processors are equipped with spell check but there remains a huge blind spot to these programs: homonyms. Homonyms are words that sound the same (or similar), spelled differently (but correctly) and are often confused for other words. Here is a list of frequently confused words and spellings.
Accept and Except 
Accept indicates allowing or receiving something: I will not accept any late papers.
Except means 'but' or indicates an exception: I will not take any late papers except in the case of illness.
Allot and A Lot 
Allot means to allow for, assign: We decided to allot fifteen extra minutes for the discussion.
A lot means many, a great quantity: We extended the discussion by fifteen minutes because there were a lot of questions.
Capital and Capitol 
Capital has several meanings. It can mean the city that serves as the seat of government for nation, it can also mean money or property owned by a business. As an adjective it can also mean "chief, principal": Washington D.C. is the capital of the United States.
Capitol refers specifically to the main building used by the government, usually in reference to the building in which the U.S. Congress meets.
Imply and Infer 
Imply means to express meaning subtly or indirectly: My joke implied that my brother ate all the cookies.
Infer means to come to a conclusion because of experience: I was able to infer than he had eaten them because of the crumbs on his clothes.
It's and Its 
It's is a contraction for 'it is': It's/It is a beautiful day.
Its indicates possession: Because of how nice the weather is, we took the dog for its walk.
Just remember whenever you want to write "it's" just ask yourself if it makes sense when you read it to say "It is"
Lay and Lie 
Lay is to put something down: I laid down the blanket on the bed.
Lie is when a person or thing is reclining: I want to lie down on my bed and sleep.
Lose and Loose 
Lose is to not win or to no longer have something you once did: By the second half it became clear we would lose our soccer game.
Loose is when something is not tight: My shoelaces came loose during the second half of the soccer game.
Than and Then 
Than is used for comparison: I'd rather have pizza than a hamburger.
Then is used to indicate a time/chronology: Today we'll have pizza, then we'll have hamburgers tomorrow.
Their, There, and They're 
Their indicates possession: Someone stole their car.
There indicates a location: Someone stole the car over there.
They're is a contraction for 'they are': They're stealing the car.
To, Too, Two 
To is used to indicate direction: I am going to the library.
Too means in addition: Are you coming too?
Two is a number, one more than one: Yes, the two of us will go together.
What's wrong with this sentence? "Two often people use one of the to other forms of 'to' too make their points."
Who and Whom 
Who is used as a subject of a sentence: Who is coming to the party tonight?
Whom is used as an object of a sentence, often follows a preposition: Whom should I invite to the party? (You are the doing the inviting in this sentence).
Who's and Whose 
Who's is a contraction of who and is: Who's making dinner tonight?
Whose indicates possession: Whose dinner is that over there?
Your and You're 
Your indicates possession: Your flight arrives at 6pm. You're is a contraction for you are: The flight you're taking arrives at 6pm.