Looks like custom ad to me. The image src is from their own cdn.. so it is impossible that they are using widget from amazon affiliate. As for being a plugin, it is not. It's just fixed text (notice the sidebar widget wrapper, it says div class=textwidget). It is totally okay to use a fixed image with link AFAIK.
Edit: The interesting bit is that how they change that ad for every page. They might well be using a custom ad rotator plugin of some sort (something which has shortcode), and then placing the shortcode into a text widget in the sidebar.