lazytom’s FeedJumbler is a web-based application that allows you to merge several RSS or Atom-based feeds into a single RSS and/or Atom feed. You can also convert an RSS or Atom-based feed into RSS, Atom and/or HTML and JavaScript. You enter the URLs of your source feeds and it generates a link to your merged feeds, which you can put into your feed reader. Alternatively you can register with FeedJumbler to get a personalized page where you can keep track of your merged feeds.

It is similar to RSSMix but there does not seem to be any limit to the number of headlines in your merged feed – with RSSMix it is 20 – and no limit to the number of feeds that you can combine. Any more than 4 or 5, though, and I think the resulting mega-feed would become rather unwieldy to scan.