1. Fuel Magnets
They claim to do something to the fuel, which is rubbish. They claim better mileage, more power, all the usual claims, and I bet it'll make you coffee in the morning as well. It is of course complete rubbish. Power increase: 0bhp.

2. Vortex Generators
There is SOME basis for these, if you own a Mitsubishi Evo, but on a smart car, they don't work, the rear of the car is too flat and vertical. They do work however on saloon cars with a terminal velocity over 150MPH. They disrupt the air flow meaning it flows smoother over the bodywork of your car, a bit like a golf ball has dimples to make it fly better. Some basis in reality, but only for fast japanese saloon cars made by Mitsubishi. Power increase: 0bhp

3. Ebay "Chip" modifications
Usually from eBay, they claim power up to 40bhp extra... Even on naturally aspirated cars, sometimes they're even adjustable. What they really do is trick your ECU into thinking the air coming into your airbox is really cold, so it overfuels, meaning you wash any lubrication from your bores and your car engine dies early. You can get the same effect with a 20p resistor from Maplin Electronics. But please, don't try it. Power increase: 0bhp + premature engine death.

4. Electric Superchargers
Also known as a 'Bildge Pump', or sometimes even a PC fan. They claim to force more air into the engine, meaning you'll get more power, which is true, in a way, but the amount of air coming from a bildge fan or PC fan is a lot less than your engine will pull in anyway, even at idle, so it's useless. You CAN buy electric superchargers, but they run from a seperate battery, and use about 100 amps to spin what is essentially a turbocharger using a powerful motor, they also cost well over £1000 to buy. Avoid. Power increase 0bhp.

5. Vortex Generators (part 2).
These vortex generators go in your air intake, and claim to make the air spin so it goes into the engine more efficiently and mix with fuel better. Which is complete crap, car companies spend hundreds of thousands of pounds per day researching the best way to make fuel mix with air in just the right ratio. You're messing with it, and you're probably restricting your air intake as well. Power increase: -5bhp for you! 

Conclusion
Possibly the easiest and cheapest way to get more power out of your car is a remap. Don't give money to people who want to sell you snake oil (which is rubbish in engines anyway, cos it's not multigrade), remaps work, they give power increases of up to 50% and work. If you waste money on products like those listed above, you're just encouraging people to make more rubbish like this.