mer·ce·nar·y -
adjective 1. (of a person or their behavior) primarily concerned with making money at the expense of ethics.
noun 1. a professional soldier hired to serve in a foreign army.
Now, you may or may not agree with the idea that staying with one team is ethical, but it's hard to dispute the similarities between "a professional soldier hired to serve in a foreign army" and a professional athlete hired to play for a different (or rival) team.
Sports mercenaries have been around for decades, many of them making a name for themselves as specialists who have no problem jumping on whatever team currently needs their skills the most. Desperate for a bench three-point shooter? How about Eddie House? Need an old guy who can give you a pinch-hit home run? Why not Matt Stairs? Need an experienced defensive point guard who can scare the competition with both his scoring ability and face? Might as well sign Sam Cassell.
The point being: History is filled with great sports mercenaries.
This list is a tribute to all of those current players who have no problem moving from team to team. This is A History Of The Best Active Sports Mercenaries.