Check out this chart from Nielsen, a market research company that monitors television viewer and web user habits. They also survey users for demographic info like political ideology—below (click to enlarge), they display the constituency of popular political and news sites by the percentage of readers who identify as liberal or very liberal (in blue) and readers who identify as conservative, moderate or undeclared (in red).

It's hard to draw conclusions from the results, though, with readers divided up so shoddily. Lumping together moderate and undeclared readers with conservatives is decidedly unhelpful, and skews the chart considerably--on first glance, it appears that conservative readers dominate mainstream news sources, while liberals are sequestered on Mother Jones and The Nation.