Have you ever gotten an e-mail where someone asked you to confirm your shipping address to insure that your package arrives on time? What about a message telling you that your vote could drastically effect the outcome of a race?
These types of mistakes are all too common. In fact, as I type this in Word, its spell-checking system recognizes that one of the examples above is wrong, but not the other.
Here is a list of some of the worst offenders I’ve seen lately:

Affect means to influence: Lena’s giggle affected her coworkers and made them laugh with her.
Effect (the verb) means to produce: Megan’s cursing effected a pinch from her mother.
Effect (the noun) is the result: The effect of the pinch turned out to be a sore arm.
Every day/Everyday
Every day is an adverb and refers to how often something happens. Call the supplier every day until he returns your call.
Everyday is an adjective, a modifier: This is my everyday outfit. Kerri’s everyday route to work is under construction.
There is a demonstrative word to show location.
Their is a possessive personal pronoun.
They’re is a contraction for “they are”.
They’re upset because their favorite neighbors are moving away from the house over there.
Further refers to additional time, quantity or degree: Laura can further explain the election process if you need more information.
Farther refers to physical distance: Texas is farther from Tennessee than Kentucky.
Insure means to get an insurance policy for, or to guarantee against loss or damage: Lynne chose to insure her bike against theft.
Ensure means to secure or make certain: Using top-notch writers and designers helps to ensure that the magazine redesign will be a success.
Capital refers to a city or money: Patrick needs to raise more capital before buying new printer cords.
Capitol refers to the building: The capitol of Tennessee is in the capital city of Nashville.
Its is the possessive form of it: The dog scratched its leg.
It’s is the contraction of “it is”: It’s not nice to talk about John that way.
Whose is the possessive pronoun: Whose car did Barbara hit in the parking garage?
Who’s is the contraction for “who is”: Who’s riding the party bus after dinner?
Your is the possessive pronoun: Let’s have the party at your house instead of mine.
You’re is the contraction for “you are”: You’re the best friend a girl could have.
Our brains trick us into hearing one word for what could be three different words. So, the rule is simple: Look it up! If your spell-checker is only going to tell you that you’re wrong half of the time, and your brain hears what it thinks is correct, better grab the dictionary and be sure.