Yahoo have released a really cool little tool called Pipes. It allows the casual user to make mashups of data to their own specifications.
Many people are familiar with xml files (this site offers an xml file, called an RSS feed, which is a datafeed, you can use a service such as bloglines to read many such feeds in one easy to use place), pipes lets you do things like combine several feeds into one, filter them for keywords of interest and output the result.
Incredibly useful. For example, if you only wanted to read posts from this site containing the word 'wibble', you could do just that! You can find images from flickr based on the latest news stories from the BBC, or perhaps have an RSS feed of china plates on ebay that are cheaper than a certain figure and are about to reach the end of their auction.
Of course, choosing the word wibble in my first example isn't that useful....
Here's a quick example I knocked up, where a French language RSS feed for this site is automatically created.