Hotels.com now features an interesting ephemeral pop-up when you view a hotel listing.
Similarly, companies like MediaCart are building cart-mounted tablet systems that can put these kinds of messages at shoppers' fingertips. As you walk the aisles, the tablet could say, "The top-selling aspirin this week is Bayer Low Dose Baby Aspirin 200 ct."
We are herd creatures, as Mark Earls has said for years. And one of the easiest ways to outsource our cognitive load—our decisions—is to simply do what others do (because if so many people do it, it must be good enough).
Research has found that this kind of 'social proof' is extremely persuasive. (Robert Cialdini is a leader in this work.) And the more similar the reference group is to you, the more persuasive the message is.
Amazon and Netflix are the kings of the recommendation engines. It's surprising that it's taking so long for brick & mortar retailers to catch on.