The AllPosters.com affiliate program lets you help them sell thousands of posters and art products. Because there are so many products in so many different categories available, there is a good opportunity to make money. They offer a tiered commission structure where you can make 25% to 30% based on your sales level.
The biggest gripe I personally have about their affiliate interface is that it takes a long time to log in. In fact sometimes you get timed out trying to log in and you get redirected to an error-reporting form. Once you're in sometimes it takes a looooong time too to get links, especially when you're doing something like creating your own storefront using a search term.
All Posters also offer datafeeds, and even feeds in different languages. In addition you can also get data via their web service- although occasionally you'll find that it's not very responsive.
Other than that the reports available are very informative. Also you can break the hits and revenue down by "tracking ids" so you know where the traffic is coming from.
Customer service is not very responsive. The odd thing is that sometimes they'll contact you and ask you how it's going or if you have any questions, but when you do reply and ask the questions you don't get a reply. Also several emails I had sent them about the web service and cross-linking were mostly no answered.