Everything you can sell is essentially a product or a service. In fact even the service is essentially a product. There are howerver some pretty creative ways to mix the two. You could for example create a must have electronic service, (software, ebook, ect) and give it away in exchange for your visitors trying out one of your affiliate products.
Ad-Aware does this with thier spyware removal software. They give away a free license for the pro version if you sign up and pay for one of their affilate products. The customer gets to choose which affiliate program to sign up for, so there is a strong chance of actually having something the customer can use. Ad-aware gets the spiff. The affilate gets a new customer. The customer gets two products for the price of one. Everybody wins.
Adding incentive to a pay per click system is generally frowned upon, but adding incentive to a pay per action system is often acceptable and even encouraged.