combine many feeds into one, then sort, filter and translate it
Now that I've lived in the Feed world a bit longer I can see LOTS of uses -- especially in a corporate settings. One example is when feed categories (streams, tags, labels etc) have their own feeds. Then one can combine well tagged posts from multiple sources into a single feed.
For example, in a few minutes, without reading any documentation, I merged two of my blogs into a single Pipe feed .
Now that's cool.
It would be much cooler if Google's Blogger supported category-specific feeds, but they don't. Other blog tools do better, I hope Blogger eventually joins the 21st century.
 Pipes couldn't auto-discover the Blogger Atom feeds, I had to enter them directly.
Update 3/4/08: Oops. Blogger can do "label feeds" , but the capability isn't exposed in the templates I use -- not sure it's in any template. I'll have to explore a bit more.