Mashing up RSS feeds together is nothing new. Yahoo Pipes has been allowing you to do it for quite awhile but there was a level of complexity that scared away many users. I know I could not be bothered. When I read about RSS Mixer on Mashable I thought it was just more of the same but I admit to being wrong.
RSS Mixer lets you easily combine RSS feeds together into one feed. For example, say you wanted to combine Reuters, New York Times, & the Wall Street Journal into one RSS feed for news. You can also later easily add feeds to the mix without having to subscribe again in your RSS reader. They even have a Firefox plugin to easily add feeds to your mix.
Check it out at www.rssmixer.com and let me know if you create any cool mixes. This is a great way to cut out some of the clutter of channels in Viigo and FreeRange on your Berry.