MediaChannel's Media Ownership Chart http://www.mediachannel.org/ownership/chart.shtml
This is a handy graphic chart of who owns what. This page is one large images, so it may take a long while to load on slower connections. Columbia Journalism Review's 'Who Owns What' http://www.cjr.org/owners/
Colombia Journalism Review provides a clickable list of the major media companies and their holdings. This web guide demonstrates the exceedingly far reach of these companies.
The Nation's 'Big Ten' http://www.thenation.com/special/bigten.html
Here's a clickable chart of the world's ten biggest media conglomerates. The 'Big Ten' shows that concentration of media owndership isn't just a problem here - it's happening worldwide.
The Incredible Shrinking Ownership Group
http://www.corporations.org/media/
In 1985, there were 50 companies who owned media outlets. The graph on this site shows that between then and now, the number has dwindled to only six, and if the planned deregulation goes through it'll decline even further. Includes the major media reform advocacy groups.
FAIR's List of For-Profit Media
http://www.fair.org/media-woes/corporate.html
FAIR points out that most media outlets are owned by for-profit corporations, which by nature makes them more accountable to the stockholders than the public interest.
NOW's 'Who Controls The Media?'
http://www.nowfoundation.org/communications/tv/mediacontrol.html
This easy-to-read chart breaks it all down - even down to which cable outlets are controlled by which conglomerates.