RSS Mixer is a service for combining several RSS feeds together to create a master feed. The result contains all the feeds you've added, and makes it easy to plug it into a feed reader or share with others using a single URL. I gave the service a spin earlier today and it does just what it advertises, if not a little more.
In addition to building your own RSS feeds, there are tools for you or others to keep track of them, including a widget for Apple's Dashboard, and an embeddable version for blogs and Web sites, which I've inserted after the break. For iPhone users, there's also a short link you can pass around that's optimized for the iPhone's resolution and touch screen. It's very simple to use, and friendly on the eyes and fingers.
RSS Mixer is by no means the first service to let you blend together RSS feeds. In fact, FeedBite, FeedJumbler, and Feedshakecan handle this function quite easily, and have been around a little longer. What sets RSS Mixer apart is the way you can share your feeds with others--as each feed gets its own page with the last 25 posts, links to each included feed, and easy ways to share the contents of that feed with others. All of these feeds and pages end up in a massive user directory which can be browsed and sorted by creation date and popularity.
The one thing that RSS Mixer is really missing is a way to register and manage your user-created feeds. Once you've made a custom feed, there's no way to go in and remove specific sites or to keep track of which ones you've made. Given the fact this is Version 0.1 and a prototype, I'm willing to cut RSS Mixer some slack, but having to find my created feed a few hours later without even a search tool was unnecessarily hard.