Use some type of back-stop and a bucket or old milk crate.
Have one player hold the ball at shoulder height and drop it straight down into the strike-zone.
The batter must see the ball and react fast enough to hit it.
This drill teaches the batter to take their hands straight to the ball.
Have the player dropping the balls, drop them at different times so the batter does not "cheat".
The batter can watch the release of the ball, but cannot move prior to the ball being dropped.
This drill should increased team bat speed.