Lint Alert, $39.50 plus shipping, "can help prevent dryer fires" by letting you "monitor and detect lint buildup in your dryer duct." It's basically a pressure sensor that you install in clean dryer ductwork. It detects the levels of pressure needed to force air out of the dryer and displays green, yellow, or red LED lights. At higher pressures, indicating poor airflow, an alarm sounds. The Lint Alert has a built-in outlet, helpful if outlets are scarce in your laundry room.
We installed Lint Alert as instructed, using a drill to create the required hole in the dryer duct. To measure actual pressure differences and assess the device's accuracy, we put a separate gauge inside the duct.
Lint Alert works. Because of the potentially catastrophic effects of a dryer fire, it could be worth buying for extra safety, but you still should clean the dryer duct periodically.