When you need something really clean you need to go with something powerful. You also want to choose something that's easy to use and doesn't require a lot of elbow grease (imagine trying to scrub the black stuff off of your concrete walk-way!).
Power washers can make any cleaning job easy. Available in gas and electric models you can find any style to fit the job you need.
If the job is a small job you want to use an electric power washer. The electric models are smaller, less powerful, but portable. You can also use them inside. If you have a larger job like the outside of a building or a large structure you may want to use a gas powered washer.
Gas powered washers are typically more powerful then the electric models. They are larger and therefore less portable. They are mainly used for commercial type projects. Also, gas powered washers use fuel to operate so they do emit CO2 gasses. The electric models, obviously, do not.
So where do you find a good power washer? You can try your local home improvement store. Places like Lowes or Home Depot may carry them. If not you can try online retailers. Sites like Amazon.com offer a vast array of products (including power washers) that vary in price. You can also find online retailers that offer free shipping as an incentive to buy from them.
So if you're out looking for a power washer consider your options and always remember that to get any dirty job really clean, nothing will beat a good, effective power washer.