Meet the Swing-OPS SpeedSensor: a baseball swing analyzer that helps you monitor your performance and improve your swings. It fits on almost any bat and has a display to show your stats. It measures your bat speed, swing power, and tightness. The battery life is good for 20k swings. You can store up to 50 swings on the SpeedSensor to upload to your iPhone at a later time.
The SpeedSensor lets you analyze and compare your swings, see your acceleration / swing path, and more. Did we mention that it has an Apple Watch app? Sets you back $99.