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I've just taken a brief look at PubSub following some mentions at the RSS Winterfest. This service takes a different angle on aggregating feeds, almost "Downse-ian" like EduRSS in that you can track among thousands (they say) of RSS feeds for particular keyword searches. And the results are presented to you via RSS!
PubSub lets you filter over one million weblogs and information streams to find the content you're looking for, in real time. It's like searching the future.
"Searching the future" is a stretch (maybe it is "RSS is here just not evenly distributed"??) but it does some to be looking at RSS sideways- rather than tracking down the sites for the sources of RSS feeds, you end of setting up custom feeds for keyword searches (curious to know if it picks up things not found in Feedster).
For example, I created an account to track the matches on "maricopa learning exchange" and the results are fed back to me via:
I can imagine teaching a class where students need to conduct research, and it is upon them to develop the best keywords (perhaps by testing results first in Feedster, and then monitoring these dynamic PubSub feeds via an RSS reader.
They already offer a few example Feeds to watch via PubSub:
RSS, RDF and ATOM (sounds like a bad law firm)
Microsoft Longhorn (sounds like Foghorn's brother, "I say, I say, I say, son, that's no way to code an OS")
Mars Global Surveyor (longer lines than Disney Land for that ride)