This has been discussed extensively on car forums, and motorcycle forums as well. You can:

- Shut off the engine

- Put it in neutral

- Stand on the brakes (all but the most souped-up aftermarket monsters will have brakes that are stronger than the engine. Please note, STAND ON THEM. If you drag them lightly, you run a higher risk of overheating them, rendering them useless)

- Use the emergency brake

- Use the clutch (if it's a stick)

- Pull all large fuses from fuse box

- Induce a spin

- Drive the side of the car into something like a divider to scrub speed

- Run into the back of another car traveling close to your speed. They will likely slow down, slowing you down.

In short, there's a whole lot of things you can try. People just don't always know very much about driving/cars, or don't think well while panicking.