Regret. It is an emotion felt by Major League teams year after year when they see a player that they either hastily traded away, released, or didn't pursue in free agency produce on a team other than their own.
2010 has forced some teams in enduring all of the above. In many instances, a team is certain that a free agent must move on. Sometimes the player proves that team wrong. Vladimir Guerrero is the best example of that in 2010, amongst others.
Sometimes a prospect is traded that could be of use to a team. Or sometimes a complete unknown puts together excellent efforts, causing his former team to shake their heads over all the irony.
Including Rangers outfielder Vladimir Guerrero, here are 10 players from the 2010 season that have made their previous teams wish that they held onto them.