The concepts behind the new Yahoo Pipes application really impress me as a "mashups" engine (to say nothing of it generally being an amazing web-based application). I've not played with this sort of app before (are there others that do something similar?), but found it to be very intuitive and easy to work with. I did wish for a few other sources (more geo feeds) and tools (manipulating item text/descriptions on the fly).
This article on Lifehacker is a great example of a basic use for Pipes -- create a single, aggregate feed of all the feeds in your life (Flickr, blog, del.icio.us, etc). Following the directions in that article is a good way to see a basic aggregation in action. Nick Bradbury (of FeedDemon fame) built another cool example, mixing the iTunes Top 10 Songs feed with a YouTube search to find videos for those songs.
I'm hoping to spend more time with this app over the next couple of weeks. I think it's got a lot of potential for creating custom news sources... and I'm sure the coolest uses are yet to come.
Disclaimer The opinions expressed herein are my own personal opinions and do not represent my employer's view in any way, shape, or form. Seriously.