Amazon.com is one of the most popular sites on the internet and sells just about anything.
Amazon has a lot of ways for you to sell their products. Contextual ads, direct links, etc. They will even build whole storefronts for you. You get a percent of the sale or a fixed fee depending on the item sold.
They have a flexible and powerful Affiliates tool that helps you build the links, ads, etc. and a useful forum to ask questions and talk to other affiliates. Many people decide not to directly sell anything and instead act as a reseller for Amazon using their storefront options.