RSS Mixer lets users mashup multiple RSS feeds into one single, comprehensive feed. If you have several disparate feeds available on your site that you want users to follow (posts, comments, individual authors) this is an easy way to get it done. Users must simply provide a title to their 'Mix' and then add the feed URL's. RSS Mixer then mashes it all up and provides several ways for you to offer it to those visiting your site - an RSS feed of the combined feeds, straight HTML, a Web Widget for your blog and even of an iPhone version of the mix. It's been getting some attention so I found the service to be a little slow today. Similar services are provided through Yahoo! Pipes and Microsoft's PopFly, but not with the extensibility.