I've been looking at the Yahoo Open Data Tables and Query Language documentation. This is truly amazing stuff! It provides a service API that accesses many well known data sources (many are Yahoo) and transforms the data into XML or JSON. The data sources can be external URLs that provide XML and Yahoo does the fetch, parse, extract and transform that you want. You can provide a definition of some other external data source and they will hook it into their unified API fetch/query/transform service.
Some of the data sources are Flickr, local listings, geo location info, web search, image search, news search, weather and so on. One stop shopping for lots of great data.
Their console http://developer.yahoo.com/yql/console/ is a great way to see what's possible.
This is what I've wanted for many years. A long time ago I wanted to build a service that would provide "XML data sources" (I even registered xmldatasource.com) for everything available on the Web - now it looks like Yahoo has actually done it.
Let's hope they keep this data access service open to all.