The most famous example of noncompete agreements in the US was the one that saw Apple, Google, Intel and Adobe agree to avoid recruiting from each other - until it was found to be illegal and fines were issued all around.
In this piece from HR Executive, Peter Capelli of Wharton provides a short summary of the rules and gives a view as to why noncompete agreements are so much more common than ones involving price-fixing - despite the obvious similarities.
Noncompete agreements are extremely popular now, with some evidence suggesting 18 percent of the U.S. workforce is bound by them. Once an employee signs one, they cannot quit and work for the employers identified in the agreement. They are legal, although not always enforceable, depending on how they are drawn up. Simply put: The broader they are, the harder they are to enforce.