Traffic programs may have a free component. However, most of the programs are paid only. The idea is to pay for traffic instead of spending your time clicking or reading emails. The reviews cover a variety of programs. Some are recurring payments, while some even offer residual traffic with only a one time payment.
Most of the traffic programs are not capable of delivering large volumes of traffic, even if you have available credits in your account. Some programs offer traffic, but cannot tell you how many impressions you will receive. Some of the programs may rely on low quality traffic in order to fill their required traffic inventories.
Paid traffic sources allow you to scale advertising that is converting. This also allows some experimentation and testing. However, you must decide how much money you are willing to spend on unproven ads.