I have a post on using spot instances in the cloud:
Spot instances are an idea which came out of Amazon Web Services. Specifically, the people at AWS realized that they had excess capacity on servers and in the cloud, excess capacity is typically a bad thing, as you’re paying for resources not in use. Going back to basic economics, when you have excess capacity, you have a surplus. There are two ways to deal with a surplus: decrease supply (shift the supply curve back) or decrease prices (move down the demand curve).
There are some complicating factors here which make it tough for AWS or other cloud vendors to do either.
Of course I wasn’t going to let a discussion of spot instances go without hitting a bit of economic theory. Just be happy I didn’t break out the supply and demand curve visuals…