Voltage Tuning Systems and similar

I was at Best Buy the other day, getting a radio installed in my other vehicle, I saw a $35 gadget that I can only imagine doesn't really do anything. Basically, it's a thumb-sized voltage regulator that plugs into the 12V outlet and is supposed to 'increase gas mileage up to 25%!' by smoothing out noise in the electrical system. My assumption is that this does nothing, or else manufacturers would have already added small capacitors to the electrical systems of cars.

This led me to search online out of sheer curiosity, and I found this, which claims to raise Prius mpg by absorbing the back electromotive force. Which reminds me a lot of the Pulse Plugs, which, I believe, a member of our own forums tested and found to actually reduce mpg in his prius.

At the same site, I found this, which would appear to do the exact same thing as the gadget I saw at Best Buy, but for hundreds of dollars more. I'm trying not to let the higher price tag make it appear more effective, but really, doesn't the prius' solid state converter already smooth the 12V bus on it's own? And would it really have that much effect on a non-prius?