FeedRinse is a web-based service that lets you filter your feeds. For the price of a free registration, you can filter up to five RSS feeds. For $4 or $6 a month, you get to filter more feeds and do more with them, depending on the level of your subscription.
Each filter can be fairly complex — feeds that match (or don’t match) a given keyword, tag, or author. So you could search the New York Times’ headlines feed for all entries that contain the word “Washington” but don’t contain the word “Congress” and are written by your favorite reporter. Your filtered feed list can then be exported as an OPML file and read in your favorite aggregator. (The filtered feeds are redirected through FeedRinse.)
What seems to be lacking from the service at present is a way to easily apply the same filter to many different feeds. But as Steve Matthews points out in his blog post on this subject, you can use other services such as RSSMix to do the building of a single feed, which then could be filtered through FeedRinse.
Feed Filters
[Thanks to Steve Matthews of the Vancouver Law Librarian Blog for telling me about this one.]