Can you provide just a little information so we don't have to type all the answers to the dozens of possibilities? What speeds does this occur? Where do you feel it; steering wheel? Seat? Whole car? Does it get better or worse if you're turning a little?
How many miles on the car? How old are the tires? Automatic or manual transmission? What engine size? Does the car pull one way if you let go of the steering wheel? If it does pull, does that stop if you let up on the gas pedal. Does it pull the other way when you're braking?