I'm not sure why there isn't a post about this already -- it's not something new and I've had it installed for quite a while.. Surely it's been mentioned before on DC, but I can't find it.
CamelCamelCamel is a free tool which can show you charts of the prices of items on Amazon.com (and now also NewEgg, BestBuy, and others.). There is a plugin for FireFox and probably for other browsers too. It will show you a nice chart of price over time, including the amazon.com official price and the best used price as well.
I think it's as a plugin that this really shines, especially if you do a lot of browsing on Amazon.com.
It will also let you create notifications so that you are emailed if the price ever drops below your target.
This tool is especially useful and fascinating for a site like Amazon, because Amazon has a very aggressive and experimental pricing system that is CONSTANTLY fluctuating the prices of items to find out how it effects behavior.
If you didn't know that already, make sure you absorb it now: The price of any item on Amazon.com is likely to change dramatically from day to day. And with this tool (and there are alternatives that are similar to it) -- you can see what the high/low/average price is of the item you are looking at compared to the current price.
Highly recommended, easy to use, free, and a funny about page -- I wouldn't shop amazon without it.
Plugins for firefox and chrome: http://camelcamelcamel.com/camelizer
Roundup of other similar tools: http://www.makeuseof.com/...-trackers-bargain-hunter/
I've only tried CamelCamelCamel so I can't comment on the others.