The most common analogy for cloud computing is that it is like utility, although I would argue that cloud computing is the analogy in itself already...

 

Here is a new one, and quite good in fact:

Take the monetary system. In his address to the annual RSA Conference on computer security held in San Francisco on March 2nd, Art Coviello, president of EMC’s security division, noted how civilisation started with barter, then invented coins to make money more portable—even though people still had to carry their wealth around with them physically. The first step in the virtualisation of wealth came with the introduction of paper money. These promissory notes, with no intrinsic value, forced people to deal with the concept of attestation—certifying that something is genuine. And with that, the advent of financial instruments such as stocks, bonds and mutual funds created ways of sharing wealth—so that when one person wasn’t using it, another could. Today, virtual money dominates the money supply. In much the same way, virtual processing will one day dominate the computing supply.