Giancarlo Stanton of the Miami Marlins hit a home run in a spring training game against the New York Mets that some are estimating to have traveled more than 500 feet.
The ball was finally stopped when it hit off the top of the batter's screen for the other field. Considering that the screen appears to be 30-40 feet beyond the center field fence and the ball hits 25-30 feet up the screen, 500 feet might be a good estimate.
Here is a video via MLB.com. A GFY can be seen below and gives a better view of where the ball landed.
Here is a GFY.