<p>What's the coolest mashup you've built?  What APIs did you use?  How was the mashup better than the sum of its parts?</p>

Although probably not a true mashup, I created a script hat would read the Apple Movie Trailer RSS feed, and then visit the links in the feed to download the HTML of the pages linked to in the RSS feed. From there, it would download the container .mov files, and parse out the actual URL to the movie you can actually download.  SO while all the information was coming from Apple, the script would download multiple things, from different parts of their site, to piece together a final RSS feed that would contain links to .mov files that actually contained the movie trailer.