Someone e-mailed this to me:
Media concentration, also known as media convergence or media consolidation, basically comes down to the fact that fewer and fewer companies own the media.
Mediachannel.org has created a comprehensive chart of exactly who owns what.
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.
This is a clickable chart of the ten largest media companies in the world, current as of Dec. 20, 2001 (it is important to note that media concentration is not only an American problem).
A graph of media ownership shows the number of corporations in control of US media plunging from 50 in 1983 to only six now. It is followed by a really useful list of links, which includes the major media reform advocacy groups.