Power factor correction devices: Can they really reduce your electricity bill? article tells that so-called “Power Saver Devices” (known by different names) are nothing but Power Factor Correction (PFC) devices that would connect to the mains and improve power factor measured by your electricity meter.
The so-called “Power Saving” device may be useful for industrial applications, where utilities charge the user based on PFC rather than real power. Such power factor correction device typically includes nothing but a capacitor that is to be connected in parallel as the additional load. The effect of these two opposing reactances in parallel is to bring the circuit’s total reactive power close to zero.
However it is important to note that utilities bill a residential user based on real power that does not factor in Power factor ( inductive load is minimum in typical home. Thus a PFC device may not reduce the electricity bill. Be careful on marketing gimmicks on this field. PFC can be useful for residential situations in the design of a backup energy system like a Home Inverter or UPS (Uninterruptible Power Supply).
Read whole Power factor correction devices: Can they really reduce your electricity bill? article to get more details. Read also article comments that correct some inaccuracies found on the article itself.
Wikipedia article Power factor has also useful information on power factor and how it is corrected. It mentions that besides power factor correction on inductive loads there are also different power factor issues with non-linear loads. Regulatory agencies such as the EU have set harmonic limits as a method of improving power factor. To comply with current EU standard EN61000-3-2, all switched-mode power supplies with output power more than 75 W must include passive PFC (built onto the device), at least.