moreover.com has RSS feeds that is based on searches through their big database of syndicated content, I have been reading their London News feed for ages but they have recently started introducing ads into their feeds. I do not have a problem with ads in RSS feeds but I think people need to think how they do it a bit more.
The moreover feeds insert the ad into the feed with a time stamp of the request time. So each time you load the feed you get a new news item for the ad. My RSS aggregator now has about 50 entries advertising travel to Ibiza. This is very annoying and I am forced to unsubscribe from this feed now, I wonder how long it is going to be before aggregators start supporting filtering duplicate entries or entries matching filter strings.

Admit it RIP , you REALLY REALLY do want to go raving at Ibiza with all the 13yr olds :-)
have a look at the filter module Dominic White (single.rucus.net) wrote last week to deal with exactly the same problem.
Thanks Barry, but it's http://singe.rucus.net/ and the sanitised feed is at http://singe.rucus.net/sanity/moreover.php and you can see the source by sticking an 's' on the end of that URL.