Mashup is a web application that uses existing web services to create interesting applications. Now a days many Web sites/applications are exposing API for web services. And many mashups have been created by creative individuals. Here’s one of the best example:
Retrievr. Revrievr lets you use image-search the Flickr photos. Just paint using the left navigation and in seconds you can see the matching photographs in the right side. Truly wonderful “Mashup”!

For other Flickr Mashups check the article on hotwired – Top Ten Flickr Mashups.