I love this question on the Biztrek blog:

Stop for a moment to consider what would happen if your business no longer existed. How would your customers react. Would you really be missed? Could they easily get what they need from somewhere else? Would they be equally well off?

Assuming you aren't Apple, Starbucks, Microsoft or Disney, how do you answer that question?

If you're Exxon, your customers will go fuel up with BP or Shell.
If you're Calvin Klein, your customers will buy Kenneth Cole or Ralph Lauren or DKNY instead.
If you're Publix, your customers will cross the street to get their groceries from Bloom or BiLo or Ingles.
If you're Best Buy, Circuit City will handle the overflow.
If you're McDonald's, Burger King and Wendy's will be happy to take over your share.
If you're Joe Shmolinski, perhaps Jim Smith or Lucy Kilgore or Mack Jackson can do your job just as well.

So what about you? What's your unique value proposition? What makes you indispensable to your customers/users/employers/market?