There are many hitting drills out there. Some hitting drills are good, but many will do more harm than good.
Bad Hitting Drills
These drills have no redeeming value and should never be used.
Level Swing Drills
Any drill that is designed to promote a swing that is level to the ground should be avoided. This is because the level swing is a hitting myth. Instead of working to develop a level swing, hitters should be working to learn how to tilt.
Two Tee Drills
High Back Tee Drills
In a conventional two tee drill, a second tee is put behind, or in some cases in front of, the target tee. The stated goal is to prevent the dropping of the barrel and/or to encourage swinging down at the ball. The hope is to create a level swing at the Point Of Contact that will result in line drives and/or prevent an uppercut.
The problem is that, although some people think that hitting the top of the ball will create backspin, in truth the actual result of swinging down at the ball is lots of (easy to field) ground balls. In fact, in all the times I've seen these kinds of drills demoed on YouTube...
...I've yet to see the ball leave the tee on a trajectory that would have it get past the pitcher's mound on the fly, much less leave the infield on the fly.
In truth, a Slight Uppercut is the desired result of the swing, and my Target Tee Drill is designed to encourage just such a slight uppercut.
Rapid Fire Drills
Rapid fire drills are ones where the hitter hits a series of side-tossed balls in rapid succession. The problem with these drills is that, if you look at the results on video, you will see that by the third swing or so the swing will get more and more top-down, which is not what you want. As a result, if you do sets of 5 or more rapid fire swings, you are spending more time working on a top-down, bad swing than a good swing.
Towel Under the Front ArmPit
Also known as the Babe Ruth Towel Drill.
Problematic Hitting Drills
These drills can help some hitters, but need to be used with caution.
The fence drill is one that has a good goal; to keep the swing compact through the first half of the swing. The problem with the fence drill is that in many cases the hitter is told to stand too close to the fence. That can cause a variety of problems, including Push Disconnection. A much better variant of this drill is the Back Foot Fence Drill.