There: A place
Their: Belongs to them
They're: They are

Your: Belongs to you
You're: You are

Then: A period of time, usually when referring to a stipulation
Than: More so

To: Towards
Too: As well
Two: Double

Where: A Place
Were: Already existing
We're: We are
Wear: Something you have on

Though: Contradiction
Thorough: Well done
Tough: Hardy or strong
Through: Going inside
Threw: Toss
Thru: Slang/Improper or a fast-food Drive-Thru

See: Look
Sea: A large body of water

Affect: Influence
Effect: A consequence of an action

It's: It is
Its: Belongs to it

Blond: Male
Blonde: Female

Fiance: Male
Fiancee: Female

"Fire!": Only use if there are guns in the universe, or when talking about cannons. When referring to bows, crossbows, throwing spears, etc., say "loose."

Tact: Diplomacy 
Tack: To attach (or the little nail you put on a corkboard)

Moot: [In America] An argument that has lost meaning
Moot: [In Britain] An argument that still needs to be solved
Mute: Unable to speak or to silence something

Peek: A quick look
Peak: The top of a slope
Pique: To stimulate

Arc: Something that is curved (usually a motion)
Arch: A building in a vaguely 'U' shape

Flout: To disregard 
Flaunt: To show off

Horde: A mass of people or animals
Hoard: A large collection of things, usually something valuable 

Pour: To dump out a liquid in a controlled manner 
Pore: Tiny holes, usually when talking about skin
Poor: Impoverished 

Rein: To control (or the strap used to control a horse)
Reign: To rule over

Advice: Information given to help someone
Advise: To give information to help someone 

Assure: To tell someone that something will happen
Ensure: To physically make sure something will happen 

Complement: Something that works well with something else
Compliment: To say something nice

Empathy: To feel someone else's pain
Sympathy: To feel bad for someone else's pain

Laid or Lie: The past tense of lay is laid, not lie. Lie is use exclusively to say an untruth 

Loose: Antonym of tight 
Lose: To misplace something or to not win

Principle: A moral or ideal 
Principal: Someone in charge or something that is important 

Who's: Who is
Whose: Belonging to someone