FeedMingle
A Single Function SiteDescription:
FeedMingle is a nifty service that allows you to merge multiple RSS / Atom feeds into a single one. To do so, you simply need to add your feeds to the designated textarea, one per line, then add a title to identify your mixed feed, and hit the "Mingle Now" button. The result is a mixed feed that you can you use to read all your mixed feeds in one place or even use it in JSON conversions or in a widget that you can add to your own site. The uses are many, as an example it can be very helpful to post to your Twitter account multiple feeds at once.
FeedMingle is the creation of Gautam Kishore of Eulogik, and you can follow the updates on Twitter @FeedMingle.