TapWidget features the QuickTap widget in Notification Center, so you can start tapping right from the lock screen.
The widget and app show at which tempo you are tapping. As soon as you stop tapping, TapWidget keeps flashing in the same tempo. In the app, this tempo can then be fine-tuned by sliding across the bottom section of the screen. This allows you to match a specific tempo of a song much more accurately than only tapping along.
TapWidget is available for iOS 8 and later.