Want to get only those stories from your favorite RSS feed that have certain words? Feed Sifter provides a form where you can paste a RSS feed, enter keywords by which you want to filter that feed and get a new feed URL.

Feed Sifter

The keywords are to be entered one in a line (multiple keywords separated by a comma will be treated with an ‘and’ condition). For example when I enter the feed URL of this blog and filter by ‘firefox’, the generated RSS feed is http://feedsifter.com/?f=http%3A%2F%2Fwww.webtechdaily.com%2Ffeed%2F&firefox

If you want more powerful feed manipulation like being able to mash multiple feeds together, filter, sort, fetch and more, Yahoo Pipes is the best web service available.