Microsoft uses compatibility and bundling as leverage to maintain their monopoly

ATT/Verizon use municipal contracts and large barriers-to-entry to maintain their oligopoly

All Monsanto has are patents on their specific crop strains. What prevents a seed producer from going into business other than the benefits of using Monsanto strains?

EDIT: ok, after googling around (the techdirt link was broken), it looks like they were bundling

http://www.organicconsumers.org/monsanto/monopoly.cfm

http://www.patentdocs.org/2008/05/monsanto-and-sy.html