Most cars manufactured today are designed to be fuel and energy efficient. Long gone are the days of gas guzzling, fuel injected horse powered speed cars. But sometimes you just get the urge to race the guy at the red light next to you, or you just get a rush from the speed of your car. You can't get that rush if your car isn't harnessing its maximum horsepower. Keep in mind that increasing your car's horsepower will definitely increase the amount of fuel you use, and not all options are cheap, but some are.
The easiest, cheapest way to increase your car's horsepower is to lower the weight of your car. You can start by clearing all the trash, clothes, and clutter out of your car. Also, don't forget about your car jack and spare tire, which are probably the heaviest things in your car. (Just a disclaimer: I feel that not keeping a good floor jack and spare tire in your car is a big no no, but if you must, then you must). In addition to clearing out the clutter in your car, you can strip it down of anything that is not completely necessary for the car to function. This includes boombastic speakers, window tint, and any other accessories you may have in or on your car.
Another cheap trick to increase your car's horsepower is to check on the cold air intake to make sure its functioning at its highest capacity. Cold air is more dense than hot air, so the more cold air you can get into your cylinder the better. The air in the cylinder mixes with whatever type of fuel you are using (just so we're on the same page, if you're trying to do this with a hybrid car, and not one that feeds off of gasoline, you may as well just stop now). This in turn leads to more combustion, which then leads to more power.
If money is no object for you, then you may want to think about super charging, or turbo charging your car. As this means you will pretty much end up having to rebuild the engine and also upgrading the stress-bearing elements of the engine, you better make sure to research this thoroughly. And make sure you're ready to shell out a couple of grand in order to do this. This is, however, the absolute best, most efficient way to boost your car's horsepower.
Tires are also something else you can work with to boost your car's horsepower. If you use shorter tires than the standard, recommended ones, they will automatically increase the acceleration of your car. You can also use tires to work around your alignment. Instead of the standard, recommended alignment, align your tires so that all four wheels are pointed straight ahead. This will definitely give you more speed.
You can also make sure that the pipes from the catalyst in your exhaust tubing that lead to your bumper are completely straight. By doing this, you will loosen up your entire system which will in turn push out the exhaust gases. This leaves more space for air and fuel, which means more horsepower.