If you blog on multiple websites and you want to offer a single feed for readers, then FeedMingle is a good service. Although Yahoo Pipes is one of the most preferred service for merging multiple feeds, FeedMingle also does the job of merging multiple feeds into one. It mixes all the feeds provided and creates one RSS feed, Atom feed, JSON feed, and a html widget to simply paste into your website or blog.
To merge multiple feeds, all you need to do is to enter the feed URLs, one in each line and FeedMingle will create the merged feed in three different formats- RSS feed, Atom feed, JSON feed. It also creates code for HTML widget which can be embedded to any website.
Apart from a feed mixer/merger it can be used as a RSS/Atom to JSON converter, RSS/Atom to widget generator, RSS to Atom or Atom to RSS Converter.
Once you have merged the feeds, you can subscribe to the new RSS feed to get the combined articles from all the blogs.
We also thought that Feedmingle was very useful so we added it to our 15 Cool, New and Useful apps list. http://crenk.com/15-new-and-free-web-apps-youve-never-heard-of/