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Mashups are not only about mixing data sources. Enormous numbers of open source resources and commercial resources are available for you to mix and integrate, saving you time. These additional resources can be scripts, libraries, skins custom components, and other elements.
Before building your mashup, it is recommended you spend time designing your application and figuring out what components and services are necessary. Then, research to find out if there are any resources you can utilize that will help you and also save you time. There is no need to reinvent the wheel if someone already spent hours creating libraries or classes you can use. Many of these resources are open source, and you can find them in blogs, dedicated web sites, and search engines. In many cases, it would be beneficial to use these open source resources. Once you create your own APIs and components, feel free to share your work with others; that's what open source is all about!