Moving aside from the plug in energy savers that don't work, I can think of ways (other than turning things off) to make a plug in energy saver that does work, with the caveat that it only works for certain kinds of loads:
* Rectifier (voltage doubled for 120V version) for PFC switching power supplies, to reduce input stage losses at the expense of lower power factor. Works even better as a modification built into the PSU in order to eliminate two diode losses.
* Buck transformer for devices that use linear power supplies, most notably older audio equipment and some LED bulbs.
* VFD for motor loads, would need to be part of a control loop.