Yahoo! Pipes BETA allows you to "rewire" the web with a nice GUI interface. With it you can build applications that aggregate RSS feeds and other services, into one masterpiece. The site lets users "pipe" information from many different sources, by dragging the pipes around. You can then setup filtering rules for how those pipes work.

Examples:
You can grab a RSS from somewhere and filter out any specified topic that you choose.

or

A typical example is New York Times thru Flickr, a pipe which grabs the NYT RSS feed and adds a photo which it finds on Flickr based on the keywords of the NYT RSS feed of each item.

The url is pipes.yahoo.com.