AllPosters.com is the web's biggest poster site, with products ranging from current movie and pop star posters to classy (and not quite so classy) photography prints. Their affiliate program is one of the web's biggest as well. If you've visited a movie-related site recently for example, chances are you've seen their affiliate ads.
The program's idea is that you direct visitors to the AllPosters website, and if they make a purchase, you get a cut. And a generous cut at that: with 20 to 25% commissions, these are some of the highest payout rates on the web. You start off with 20% comissions for monthly sales of up to $500. The more you sell, the more you make, with 25% comissions after $20,000 per month. Now most sites won't be doing anywhere near that level of business, but it's nice to have goals.
Actually, even with high traffic, depending on your audicence you may have trouble earning the minimum monthly payment - $20 for check payments. The main problem with this program is its popularity. There are a lot of sites trying to sell these posters to their visitors, and chances are, some of them are bigger and more popular than yours. However, it *IS* possible with the right demographic. This program would obviously work better for sites targeting college students than those offering investment advice.
The number of linking possiblities is huge. You can make static links to particular posters (or even something as untargeted as a "buy posters" banner), or have links that relate to particulat searches, and even mini-stores inside of a customized IFrame, so that they look like a part of your page. (The latter option is very useful because the content of the store changes as some posters become more popular.)
AllPosters also has a referrer program that's pretty generous: not only do you get 5% of all your sub-affiliates' sales (note that's sales, not earnings), but they also give you $2 when someone you refer puts up their first link, and pay $3 and $5 for the first and second sale respectively.
In the end though, your success with this program will depend completely on your site's audience. If you attract a lot of people in their late teens and twenties, this program may very well bring in a lot of extra chash.