Amazon's affiliate program, known as Amazon Associates, lets you earn a percentage of each Amazon sale you direct from your website.
I've been an affiliate for a year or two. What's interesting is that it doesn't matter what specific product I link to — if the person who clicks through to Amazon from here or there does a little Amazon browsing, and then makes a purchase, I get a cut (roughly 5% — doesn't affect buyer cost).
What's more interesting is that I can see what items have been purchased through my referrals, and also how much money was passed my way.
All-in-all, roughly £50,000 worth of Amazon purchases have been made through my affiliate links, so I received a couple of grand in the process. Not exactly in the same league as Darren Rowse, but I'm not sniffing, and hey, if I can help designers look smarter along the way, brilliant (£0.23).