Amazon Associates is a pretty good program that you can integrate into any consumer-facing site. When the Facebook app platform first came out a few years ago, I whipped up an app that integrated with Amazon associates in about a week's worth of time. Almost no maintenance since then (save for changes to reflect Facebook UI/platform upgrades), and it probably averages ~$40-50 a month, and makes several times higher during holiday season.
Amazon's Product Advertising API provides access to lots of their data, so if you so choose you can make mini niche shopping sites with significant dynamic data.
The only thing I'd be concerned about is receiving payments if you're not in one of the Amazon locales. Don't know if you need a valid tax ID or something. I think you can choose to get paid by checks though, if that works for you.