Use The Tracktor to track Amazon price shifts
The Tracktor is a very simple website, letting you view the Amazon price history for any product on a neat (if small) graph, and set custom email alerts for when the price (used or new) reaches or drops below a certain threshold.
The particular graph you see on the screenshot is for an MS Natural 4000 keyboard. That's a "lifetime" graph for the product -- I can't believe someone was trying to sell a used one for $125 back in 09. Maybe it was a cool, Colemak-friendly one like I posted on our Facebook page (in which case, the price is totally justified!).
The "Movers" section of the site shows products which recently had steep price drops. It seems buggy, though; it thought it spotted an 8TB NAS device from Buffalo Technology for $2, which would have been a drop of $2,519 off its previous price. Clicking through to the product, its price was listed at $2,440 -- still a discount, but not exactly the same.