I have a couple of old High Fidelity MC 0.5 power outlet thingys that use some sort of magnetic principle to clean AC that sound like beefed up versions of the GreenWave products mentioned above.
They were all the rage a few years back. Haven't tried any comparisons once I got my AQ Niagara 1000 but I still keep them in on unused outlets.
All the best,
Nonoise