I've been for a while considering a web service where user could select from a list the news services (or other feeds) she wants to use and they would be aggregated to a single list. The catch is that they would be piped through a bayesian spam filter that would categorize them as ham, spam, and unsure (and spam wouldn't be shown). User could then teach their own filter to show only relevant content. This hopefully would scale quite well as the content could be fetched and saved once and only status information would be saved for the users. Categorizations could be done just in time or maybe as a back process to even the load. The aggregated list could be available as a feed as well as a web page.
This is not a new and original idea so I tried to search for this kind of service or program, but what I found seemed to be only in proof-of-concept level. If I would like to read the feeds as emails this wouldn't be any problem and it may be that I will opt for that solution. There also seems to be readers/aggregators that support this kind of filtering, but not so many services that provide the results as a feed. I want to be able to read the results using system of my choice.
It's of course possible that I failed my Google-fu. Please tell if you know something about this kind of services already or have ideas for better scoring algorithms or such.
In the end I will probably implement this just for me with small effort. Easiest would be probably a combination of rss2email, spam filter and some email to rss program. It would be nice to have links added to the posts for easily marking unsure posts as spam or ham and correcting false negatives, though. The newest version of SpamBayes, by favorite spam filter, has XML-RPC interface which might make it rather easy to implement this. I'll tell if I get anything done, but don't hold your breath.