FeedShake looks like a very useful service for merging, sorting and filtering multiple RSS feeds. You give it a set of feeds and it produces a URL for a new RSS 2.0 feed. You can optionaly filter the feed items, selecting only those which have or don’t have certain words. Once interesting design decision is that they don’t really parse the input feeds, but rather just look for <item>…</item> blocks in them. I don’t know if this is a good idea or not. I guess the onus is on the users to start with reasonable input feeds.