Mythbusters debunked a few of the fuel saver devices: the magnetic variants included. Episode 53 if I remember right. If magnets or vortex generators worked, don't you think that manufacturers would include them and tout the fuel efficiency on that model?
Browns gas generators (hydrogen from electrodes in water) do work on older engines, mainly because they increase burn temperature, slightly improving engine efficiency. They may erode plugs. Air is the real fuel in an engine, sonsuming up to 3 litres every 4 revolutions of the engine (depending). Adding a miniscule quantity of hydrogen (a fraction of a cc per revolution) is not going to halve fuel consumption and overall impact is also miniscule. You'd be better off fixing re-tuning the engine.
Stromberg high frequency converters raise the energy dissipated across spark plugs and do work on some older distributor cars by more completely burning fuel, but they tend to fry electronic ignitions and man char Bougey cords.
There are only a couple of methods to realistically save fuel. Most of this is practical advice, obvious really.
1) Correct tyre pressure and accurate wheel alignment and proper balancing. Alignment depends on having suspension bushes and joints within acceptable seating tolorances. Silicone additives to modern tyres also drop consumption by a small degree. Try <http://www.tirerack.com> for tyre advice.
2) Make sure your injectors are clean. Either get them checked out, or run a bottle or two of suitable injector cleaner through on two successive tanks.
3) Make sure car is serviced regularly.
4) Do your shopping on your regular commute route, rather than going out of your way on dedicated trips. A cold engine can consume up to 40% more petrol above average until operating temperature is reached.
5) Use cruise control in commute traffic:- smooth engine management vs. heavy stop/start foot. The point is consistancy. Try this: average your commute speed; you'd be surprised to see it's only around 40-50 km/h. If you insist at driving 130, then do so consistantly over the journey and you will save petrol overall.
6) Switch off aircon in cool weather.
7) Drive with windows closed (saloon) as far as possible and ditch roof racks. Anything above 80 km/h and wind resistance in a car starts increasing quite a bit.
8) Fill up once tank reaches a quarter full and always fill up the tank completely. Less time spent at petrol stations means less fuel used.
10) If you are idling for more than 20 seconds while stationary, you are wasting fuel. Switch off your engine at timed robots (a quaint South African term for traffic lights).
11) The rumours of filling up in the morning at petrol stations (more dense fuel), or only filling up two days after the tanks are refilled (to make sure sediments settle in their tanks) are urban legends.
12) Drive like a local, not a yokel. Time the robots in your area and drive smoothly and you will easily out-pace the local "tourist" or dozy driver.
13) Ditch any heavy items in the boot. Anything that shouldn't be there shouldnt have to commute with you day in and day out.
14) car pool, but make sure you're doing the driving
15) Those prone to road rage consume up to 25% more petrol than calmer drivers. Zen, calm, karma and all that.... Oh forget it... Just leave the BMW lane open.
You don't have to drive slowly, but drive smart.