I haven't had a chance to take a look at it yet, but KnowNow (where I worked for a few months) has released under Apache-style license some of their technology. Matt's got some thoughts about it.
A timely application I'd like to see with it is a page of Amazon's top selling items during this Christmas season, perhaps broken down in minute or 30-second segments. I.e. every thirty seconds, whatever sold the most items would appear on the page. I'm not sure whether this is possible b/c I haven't looked at what mod_pubsub actually does yet, and also because I doubt Amazon crunches and releases their data that quickly. I don't even know whether you can get the most popular items (updated hourly) via the Amazon API. But you get the idea. It could be a cool holiday gift tracker widget.