The Invisiblehand API brings this cleverly named browser add-on focused on finding the lowest price for something that the user is shopping for to a wider audience. Constantly updating prices on millions of products from books to air fares, Invisiblehand stands out for its ability to learn about new products it hasn't seen before. The documentation indicates this is a RESTful API with JSON responses.
One critical ability for any app like this is the ability to match products accurately, which it does in part through identifying UPC symbols, ISBNs, EANs and more.
Invisiblehand offers several use cases as examples, including searching for products through smartphone apps, real time price availability for e-commerce web apps, and a shoping browser extension backend.
With 696 retailers and 589 airlines particpating in 3 countries, they've put up a counter on their website of the amount of saving's they've provided, now topping $1 billion. Retailers include the usual (giant) suspects like Amazon and JetBlue.
The user doesn't even haev to visit a retailer site for the app to work--Googlng products brings up the cheapest through Invisible hand, for example.
Now you can shop until the prices drop.