Nat Torkington wrote an interesting piece titled, "Six Basic Truths of Free APIs."  He made six suggestions worthy of reprinting here:

• Free APIs are not a god-given right. Businesses offer them for their own self-interested reasons. If you build on top of the API but aren’t delivering the value for the business that provides the API, your use of the API will probably go away.
• If you build your own business on top of an API, you need a contractual relationship to ensure the service doesn’t get taken away from you. These generally cost money.
• If you find a way to get something from a site that isn’t explicitly offered as something for you to build on, your use of it will probably be fought unless you’re delivering value as in (1).
• he provider of your API will find it easier to implement services on top of their API than you will. Therefore you have to add something of your own that’s difficult to replicate, something beyond a simple UI tweak or a feature like "search", so that the business that provides the API doesn’t simply compete with you when you look like you’re succeeding.
• For these reasons, free APIs are a very poor substitute for having the source and the data and thus owning and controlling every piece of your application.
• For these reasons, there’s no such thing as a free API if you’re looking to build a business.