To give this list some context, we need to look at the history of free agency. San Francisco Giants greats like Willie Mays, Willie McCovey, Mel Ott, Christy Matthewson and others were all technically free agents when they were signed by the Giants. However, there was also no draft yet and players could not leave teams for another unless they were traded or sold. The players had no say in the matter.
Then, in 1969, St. Louis Cardinals outfielder Curt Flood became the first professional athlete to fight for free agency rights.