Affiliate marketing is a very popular practice on the Internet. Under this system you have a merchant that is willing to let other people (the affiliates) sell directly or indirectly its products and services, in exchange for a commission. Sometimes this type of advertising is also called CPA (cost per action) or CPL (cost per lead) based.
Affiliates can send potential customers to the merchant using several tools, from banners to text links and product reviews.
In order to find suitable affiliate programs you can turn to individual companies and publishers like Dreamhost and SEOBook, or join affiliate marketplaces and networks.
List of popular affiliate marketplaces and networks: