Interest in a gold economy
Imagine a society on a gold standard where the supply of gold is fixed. As the economy grows, more goods and services become available. Because the supply of money (gold) is constant, gold constantly rise in value, and prices of everything drop steadily.
Economists of today would call this "deflation", and some of them would claim that it's a Bad Thing. I don't think it would be a Bad Thing--seems perfectly natural to me--but that's not the point of this post.
Imagine further that you lent someone an ounce of gold. How much interest would you charge him?