Keepa.

It shows a graph, under every Amazon product page, showing what price changes have gone on in the past several months. There is also options to be alerted when the price goes to an amount you can set yourself.

Easier to get the cheapest prices on Amazon, if you don't mind waiting sometimes. Sometimes you genuinely find a product that's the lowest price it has been in years, but at least with Keepa you can see that for yourself.