A big part of what goes into a mashup is the data sources that are used to create the different components in the application. Plenty of fee-based services are on the Web, but unless the mashup supports an application for a large corporation the price tag can be unrealistic for most individuals and small companies to pay. For those groups of people, it is better to try to find free or open source services to provide the data for any application that is to be built.
A lot of times, finding the data sources for an application can be harder than coding it. The availability of data is only as good as the services are at advertising it to the world. Even data that is publicly available, or is at least supposed to be in the public record, can be buried within pages and impossible to find. Also, a lot of publicly accessible data is available only for a price.
Being able to factor these variables into the budget for a project can sometimes keep the project from being canceled completely. It is important to know what needs to go into your mashup.
When it comes to public data, a wide variety of information is available, from demographics to death records. A lot of this information comes at a cost, though some government agencies are beginning to allow access to some information for free. The idea that it is public can excite many developers before they realize that the data comes at a price. Remember that there is a clear distinction between public and free—they are not ...