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.