AJR's Newslink: Guide to US and Canadian Magazines
Broken down by category.
Columbia Journalism Review
"The Columbia Journalism Review is recognized throughout the world as America's premier media monitor—a watchdog of the press in all its forms, from newspapers and magazines to radio, television, and cable to the wire services and the Web." Website also includes tools for journalists.
Editor and Publisher
A comprehensive journal covering the newspaper industry. The website provides many articles, but be sure and check out the libraries' subscriptions to Editor and Publisher where you can get the full-text of articles that are not available at the free website.