Amazon. This is the most well known of affiliate programs. They pay you a percentage of sales (5-15%) that are generated through your site. If you are advertising a specific item you still get paid if they buy something else on that visit. Amazon isn't just books anymore. You can also sell electronics and toys now. The best thing about their program is that you can find items that are a perfect fit for your website (which actually adds content to your site), and it's not as hard to sell something from such a well-known site that carries many extremely popular items. You still usually need quite a bit of traffic to earn much. But you can get high click-thru and purchase percentages if you target certain products to fit your visitors. I have had people click through at a rate as high as 8-10%.
Barnes & Noble. The biggest book-seller offline also gives you a 5% cut of what you sell through your site. I don't use it simply because I like Amazon better, so I can't make a fair comparison.
CD Now. This is another program used by many. I haven't used it, but I listed it because of its popularity. And music sells.
