A servo tester. This allows you to control the ESC without an RX/TX. Though in some cases using a TX that has 'test modes', or delay timers, can allow you to run a motor up, and down, to a pattern - useful for balancing in some methods.
But a servo tester is useful to activate retracts... test servos (of course).... so it is very useful for a fair number of reasons.
Pretty much a 'must have'. Which the Power Meter is also.