Giving away a free informational product such as an e-book, an email series or a mini-course is a popular tactic many affiliate marketers use. Usually, your readers will have to provide their email addresses to receive the product from you. You can then use this to sell to them via email marketing. Additionally, an informational product can generate interest in the actual product you're trying to sell. If your product is popular enough and brings enough traffic to your site, you could also monetize the traffic in other ways, such as AdSense.
Although this article has been written in 2007, now in Feb 2014, I still find it full of great advice and tips. What I learned from this article is that when writing a review I should focus on how this product helped me achieve X instead of simply repeating and enforcing the features and benefits of the product that can also be found on the sales page.