Here's an interesting mashup concept: Smart Editor, a rich text editor that uses web APIs to gather potentially relevant data from the web as you type. It gets web search results to inform you about related stuff on the net, related news happening around the world, Flickr photos to help you visualize, and Amazon product recommendations. It is built with the YUI Toolkit and four APIs: the Flickr API, the Yahoo Term Extraction API, the Yahoo Search API, and the Amazon E-Commerce API.
This mashup won the "Best Self Expression hack" at the Yahoo Hackday in Bangalor India earlier this month with judges that included Yahoo co-founder David Filo. Yahoo has had good success with their Hack Day events (see our earlier coverage on Yahoo Hacks Become Yahoo Products) and continues to do so with this international edition.