Little known or useful Web Services we all should know about

YQL - Yahoo provide a tool that lets you query many different API's across the web, even for sites that don't provide an API as such.

From the site:

The Yahoo! Query Language is an expressive SQL-like language that lets you query, filter, and join data across Web services.

...

With YQL, developers can access and shape data across the Internet through one simple language, eliminating the need to learn how to call different APIs.

The World Bank API is pretty cool. Google uses it in search results. My favourite implementations are the cartograms at worldmapper.
_{(source: worldmapper.org)}