Arizona Fall League

The Arizona Fall League is an offseason winterball league, created by Major League Baseball as a finishing school for elite minor league prospects, and is an easily accesible league for management and fans alike.

Read more about the Arizona Fall League on the official website's about page.

Here's a brief excerpt from the website:
The roots of the Arizona Fall League go back several years, when the Major Leagues wanted to create an easily accessed offseason league. A concern was that some of the best ballplayers left to play winter ball out of the country, such as in the Caribbean and there was no way to monitor them.
If Major League Baseball created a league that it could govern and monitor, it would be better organized. If a player was injured, proper care and treatment would be on hand. With the Arizona Fall League, managers, coaches, scouts and league officials could participate.
"That was the beauty of this," says Steve Cobb, AFL executive vice president, "and that seemed to be one of the unifying factors for all the clubs. They created a Fall League where all of their personnel would be involved, with reduced travel and immediate accessibility on the part of scouting."
"There's a two-pronged mission to the AFL," Cobb says. "One is for the players to accelerate, and hopefully jump a classification. The other is for the managers and umpires to develop."