Seeking feedback on our API - a Swiss knife for semantic apps & mashups (blog.headup.com)

This API is designed for easy querying of a selected collection of useful Web Services, Wikipedia, Linked-Data, and the unstructured web. The API also provides a flexible templating language, for easy creation of semantic mashups and data intensive applications, directly from your browser.

We'd like to know what do you think are the coolest applications that can be built with this tool. What would be a cool API to mashup with. And what are your biggest challenges when building mashups.