Electric 'superchargers' will not provide boost on an automobile engine and will most likely rob a good running engine of high-RPM power by RESTRICTING airflow.

Consider for a moment what a supercharger must do to increase power. It must actually PUSH more air into the motor than it draws in naturally.  Creating boost means moving a large volume of air (CFM) at increased pressure (PSI.) Most of these on eBay are nothing more than bilge exhaust fans designed for use on inboard boats.  They might *seem* to move a lot of air but you honestly have no point of refrence, you don't know how much the motor sucks naturally.  I assure you, it's substancially more than a plastic fan can provide.

Small true superchargers like the Eaton M-90 push only a small volume of air for a boost application. However, this small supercharger will push MORE boost than a commercial quality, gas-powered, backpack leaf blower and I'm sure we've all seen what they can do. Toss an electric vent fan like these into the comparison and you have effectively brought a rubberband to a gunfight.

How can an electric blower actually rob power?  Simply put, the electric fan cannot push as much air as the motor demands at high RPM so the engine has to not only suck air but it also has to pull the fan faster than it's already moving. 

Notice that few of the electric 'supercharger' auctions will provide the Boost @ CFM rating of the fan and that's for good reason. Simple math can determine the point at which they ROB power from YOUR engine if you simply know the size of your engine.

If electric fans could boost a car engine, manufacturers would be doing it.  To increase power on a budget consider underdrive pulleys or a K&N air filter or an upgraded muffler/catalytic converter setup.  Simply cleaning the junk out of your car and reducing weight will have more impact on your performance than a bilge fan.

11-2006 Update

Had a couple (literally 2) people try to convince me that electric superchargers are possible.  No need, I don't argue against it being possible.   Let's try to picture what this thing would have to look like... The device will have power wires the size of a grown man's thumb. It will require charging system upgrades (alternator, battery and wiring at least.) The housing will be made not of plastic but of precision ground aluminum and steel.  The electric motor mounted to it will be larger and heavier than the electric motor that starts the engine (the device will weigh about as much as a toddler.)  The fan within will not resemble a hamster cage in any way and it will certainly not be available on the shelf at West Marine.

What it won't look like:  Search eBay for 'electric supercharger' There's even one for $279 that's patented! One whole horsepower? *rolls eyes* Seriously, the blades look like they were cut out of a soda can and I have bigger wire going to my tweeters. Hey, at least they gave that one a cool name.. Wish you luck.

Sorry, that's just the way it is. Using multiple power wheelchair motors and 1/2 of a turbocharger, a brilliant DIY guy/gal might be able to build a device to give boost to a very small car engine. Getting it sufficient power will be another issue entirely and up to you.  If anyone does this, I'd love to see it and I'm very curious as to what the investment was.  As I said, if it could be done (practically) the manufacturers would certainly be doing it.