Yahoo! pipes does seem to offer the desired functionality. My first impression, though, is that the interface is too mouse heavy (admittedly my main complaint about everything that requires the mouse). I should be able to create a filter without drawing a circuit diagram. And I worry that I won't be able to keep using it without having to tell Yahoo! my password every other week.

http://www.filtermyrss.com looks much more to my liking, although it's a bit low on features. My feeling is that it's about a 10 line perl script: fetch xml with LWP, apply regex, print.

I've actually been looking for something like feedity, but I need control over the code, since I'd want to use it for sites that require authentication. However, my feeling's that this one's about a 20 line perl script with LWP and Algorithm::Diff, so it's not a big deal.

Maybe I'll combine them into a 30 line perl script and get rich!