Serval Tracker not only shows you the price history of an item (giving you an idea of its past lows) but you can set an e-mail alert for the moment it drops below a price level. Useful for this coming Black Friday.
The link below goes to their website, but you can also get a 3rd party Chrome extension at https://chromewebstore.google.com/detail/serval-tracker-linker/lgkaeblmlapcmgmnjnddefdinlcmpmnk and a Firefox extension at https://addons.mozilla.org/en-US/firefox/addon/serval-tracker-linker/.