Years ago I wrote this silly little Mashup example:
It grabs an RSS feed, in this case the top daily search term % movers from http://buzz.yahoo.com/feeds/buzzoverm.xml which gives you an indication of what is on the minds of web searchers right now. I took these searches and did a Yahoo Image search and a News search and combined them in that oval interface you see. I had to do a bit of RSS and XML parsing to take these different data sources and combine them. This is what Pipes is all about. It provides a visual environment for manipulating data sources and then provides a number of different ways to get the results and integrate them into other things. Directly in your RSS reader is probably the simplest, but you could also feed it to PHP and do further data manipulation.
A simplified Pipes version of the above takes the same Buzz.yahoo.com RSS feed and does a Flickr search on each search term. The result looks like this:
The point here is not the visual output. It is meant to be fed to something else. Hover over the "Subscribe" link on the right there. Then click on the "How this pipe was made" image on the left to see how it works.
This is a particularly lame and simple pipe. Some much cooler ones include:
Blog Buzz for Pipes combines a couple of different blog watching feeds, filters out duplicates and gives you a combined feed in reverse chronological order. When you look at how it was made it becomes immediately obvious what it does. You can save a copy and make your own version that watches for whatever terms you want.
Another interesting one takes the New York Times front page, runs a content analysis on it to get a set of representative keywords and then does a Flickr search on each of those. http://pipes.yahoo.com/pipes/vvW1cD212xGMiR9aqu5lkA/.
Here is a much more complex pipe that takes some user input. It finds apartments near things. In this case it looks for apartments within 2 miles of a Park in Palo Alto, California by searching Craigslist, then doing a location extraction and then doing a Yahoo! Local Search for that location.
Even if you have no use for processing data sources this way, open up one of these Pipes and drag the boxes around and watch the pipes react. Web apps don't get any cooler than this right now.