Has anybody used these? There are a few different models available. While they don't remove calcium and magnesium compounds, they do keep them in solution while they go through your plumbing and out the showerhead, washing machine, etc. They are claimed to improve the effective 'softness' of the water due to this increased solubility.
Here are a couple links electromagnetic and another that is electronic/catalytic.
Before you dismiss it as nonsense, look at the government evaluation of this technology. It does appear the focus is on de-scaling of pipes that build up lime deposits, especially under heat. Kind of like the average home hot water heater . . .