For the anal retentive audiophile...a mains frequency tester

I don't know what good it would do, given that there is not much you can do about the frequency and any variation would have little effect--it would really only affect a turntable with an AC synchronous motor, and even there, any variation would be so small as to be inaudible.  Save your money to buy something more useful.

I agree that turntable speed control boxes, if implemented correctly, can reduce the small momentary variations in line frequency and otherwise clean up the power being fed to the turntable motor.  I employ the Basis power supply on my Basis table.  It does help, though the effect is quite subtle, and it allows for easier speed change.  But, I don't know to what extent the improvement is from frequency stabilization, and I have no idea how a meter measuring frequency variation would be useful; I have no idea of what measurement values would indicate a need for a motor controller, so it is not even of use in deciding on such a purchase.