They've also kind of monopolized several crops by developing "Round Up" resistant plants. If you farm non-monsanto seeds and your crop's seeds blows into a "monsanto" owned farm they have been suing, and winning, thus forcing independent farmers out of business. It started with soy but now I read where they're starting to do it with corn as well.
The documentary Food Inc. even goes as far to say that they sometimes sue small farmers to force them into bankruptcy. Even when they would lose if it went to court their lawyers tie the case up to the point where the small farmer can't compete financially. The legal fees get so high that they go bankrupt or sell.
Ooops. I have this backward. When their seed spreads to the neighbors farm through "acts of God" they are suing the farmers for using their seed. That makes somewhat more sense.