Anyway, surprised no one mentioned the banking sector which would have prevented the 2007 crisis. It being the eighties, though, no one was going to break up the funding that made greed good.
The problem there was mostly that banks were working as retail banks and investment banks at the same time - i.e., they were both making small loans to individuals
and trading in shares and other commodities (futures, all kinds of derivatives, etc). This had been forbidden in the Glass-Steagall act, passed shortly after Black Tuesday (because people decided that this was one of the major causes of the Depression). Of course, this is behavior also somewhat trust-y, since this allows/encourages mergers between the two varieties of bank.
The act started getting amended into oblivion in the 60s, and was eventually repealed entirely in 1999.
Whoops.
Anyway, banking (of both varieties) has never been quite a monopoly in the US. An oligopoly, sure, but as far as I know fairly competitive and not too cartel-y, especially with the need to compete sometimes with local credit unions and such.