Today, Kevin Youkilis celebrates his 34th birthday, but for the first time in his career, Youk is going into the start of the MLB regular season following his big day as a member of a team other than the Boston Red Sox. And even worse for any BoSox fan, as an infielder for their hated rival, the New York Yankees. While many athletes can get a pass since some circumstances are out of the control of the player given the fact that sports is a business, this decision by Youkilis was his choice. But he's not the only one to pull off such a dramatic stunt. 

From Jerry Rice making the unthinkable leap from the 49ers to the Raiders to Roger Clemens taking up a roster spot on nearly half of the American League East, here's a look at the players that joined their arch-nemesis in Athletes Who Joined Their Sworn Enemy