Plagger is a perl-based, open-source feed routing system, or so called "mash-up creator." A couple of interesting sites, like CDTube (a mash-up of Count Down TV and YouTube) are powered by Plagger and it has been driving geeks to do some interesting bits with RSS/Atom/iCal feeds.
The most annoying thing about Plagger development, for me and for end users as well, is the lack of the document, and the lack of pretty interface. And it requires half of CPAN to run it. (Well, not really, because most of CPAN modules are required by "plugins" and you don't actually need to install all plugins. They're all optional.) These barriers have been sort of intentionally made there, to reduce the S/N ratio in the community, and looks like it's been working well, but it caused the other problems ("Plagger is HARD to install!").
But anyway, the most frequently asked question in the conferences have been: "Are there any hosted version of Plagger that I can use, without installing it by myself?" and I always answered "Yes, there could be, but if you're asking me to do it, I won't. Why? I don't need it :) Plagger is licensed under Artistic/GPL and there's no way for me to stop someone to create the hosted Plagger."
So now, Yahoo! Pipes could be the answer for these people. I'm not saying "Y! Pipes ripped off my idea!" since there are other services already like Dapper or xFruit, but having that cool IDE and the feature to share the "pipe" is really awesome. Congrats to the Pipes team.
It also makes me grin that they named the service "Pipes," and follks like Jeremy or Tim O'Reilly are saying "RSS is the pipes for Internet!", which coincidentally matches with the Plagger's tagline "the UNIX pipe programming for Web 2.0." As you can see from the slides (esp. in YAPC::Europe and XML Developer's Day), I've been saying "Plagger allows you to mash up feeds just like Unix pipe/filters." Thanks to Yahoo! for proving that my vision's been correct :)