There are thousands of web resources with data that is useful to other web applications. Government sites, for instance, provide public domain information, such as the voting records of elected officials, geologic information, and rainfall history. If your application needs data that's available from other websites (and of course, if you have permission to use it), then you can easily use the .NET networking classes to scrape these sites for the usable data.
This article gives an overview of the WebClient networking class. It provides a number of demos that retrieve data, and uses the regular expressions classes to parse the data into useful information.