I know that it covers well-trod ground, but the study "What you know depends on what you watch" (PDF) from Farleigh Dickinson shows yet again that NPR listeners are the best-informed, and Fox viewers are the worst-informed:
People who watch MSNBC and CNN exclusively can answer more questions about domestic events than people who watch no news at all. People who only watch Fox did much worse. NPR listeners answered more questions correctly than people in any other category.
As these two graphs indicate, the Fox audience is even less informed (believe it or not) than the no-news audience:
Fox: when you want to know less than nothing.