I'm most familiar with baseball in the United States. Each of the four major league's in the United States has its own history on the development of free agency but baseball is the oldest league. As well as, every league has its own collective bargaining agreement between the league and the players association.

Baseball originally had the reserve clause which player's rights were retained by their team each season. The only way a player could change team is if their rights were sold or traded to another team. Otherwise the player could opt of their contract if they sat out of the league for a year.

There were a few challenges to the reserve clause over the years. One such example was with Nap Lajoie in 1902. He played for the Philadelphia Phillies then switched to the Philadelphia Athletics. The State Supreme Court of Pennsylvania upheld the reserve clause and Nap would have had to sit out a season. However, the ruling was only enforceable in Pennsylvania so he moved teams to the now Cleveland Indians.

Baseball considers the turning point when Curt Flood was traded by the St Louis Cardinals to the Philadelphia Phillies in 1970. He refused to be traded to the Phillies. Flood challenged the reserve clause. It ultimately went up to the US Supreme Court, Flood v Kuhn, where he lost. Flood then sat out the 1970 season instead of playing for the Philadelphia Phillies.

More players challenged this ruling and the reserve clause in 1975. An arbitrator reversed the decision and voided the reserve clause. Eventually the league and the players association came to agreement on when a player becomes a free agent.

There are a lot of rules and stipulations on becoming a free agent in baseball. As well as other challenges to the reserve clause such as the creation of the Federal League in 1913 throughout baseball's history.