FeedMash combines multiple input web feeds into one. You specify input feeds (RSS/Atom/JSON), FeedMash monitors and downloads them periodically. Then it combines all of them into one single Atom feed and serves this final combined feed via HTTP. You can use any feed reader to subscribe to this resulting feed.

FeedMash also can be used to subscribe to YouTube channels (even without an account). YouTube channel feeds are a little different: the links to them are hidden and also require some post-processing to be properly displayed in a feed reader. FeedMash uses YouTube channel URLs to generate feed URLs, and then it does the needed processing automatically.