Plagger is a great tool for doing interesting things with RSS feeds. It’s written by Japanese Perl Monger Tatsuhiko Miyagawa (with help from a large supporting cast). If you want to get a feel for what it does then the slides from the talk that Miyagawa gave at YAPC::NA are a good start. Over the next couple of months he’ll be giving similar talks at YAPC::Europe and OSCON.
I haven’t yet got fully to grips with all of the cool things that you can do with Plagger, but what I have been doing is to use it to build blog aggregation web sites (which people have started calling “planets”). I reckon that it currently takes me about five minutes to set up a new planet - and most of that is finding the list of feeds to aggregate. Recently I’ve set up planets for my local area, UK MPs’ blogs and a “personal planet” that contains links to various web feeds that I create. Yes, I realise that they all look the same, but I don’t really expect people to visit the web sites as they all produce both RSS and Atom feeds that you can add to your favourite feed reader.
All in all, Plagger looks like a very useful tool if you’re doing any kind of work with RSS feeds. And I think I’ve barely scratched the surface of its power.