Found 4th Sep 2017
What is InvisibleHand?
It’s a browser extension which discreetly notifies you if the product or flights you’re shopping for is available more cheaply from another retailer or travel site. The notification provides a convenient button which links you straight through to the relevant page on the competing website. It also provides a drop-down menu, so that you can see the prices at the other sites which sell that product or flight.
This has always been free but I thought it'd be worthwhile sharing it to help others out.
Here is an example from amazon: