There's an actual app called River of News for the iPad that does it quite well, but no Mac version, sadly. NNW can be configured to work this way, but it's a

There's an actual app called River of News for the iPad that does it quite well, but no Mac version, sadly. NNW can be configured to work this way, but it's a bit clunky. Everything else I've seen seems to go with a standard Mail-type setup.