What are you *really* competing with?

Dan Bricklin (co-creator of VisiCalc) has always been a hero of mine. So many assumptions of current software come from his simple innovations. His key focus was simplicity and ease of use. I love this quote from one of his business school professors, giving him feedback on his prototype:

You're competing against the back of an envelope. It's got to be really easy to use.

-- Source: Founders at Work

What is your product competing with?