Been using camelcamelcamel.com for this. They have great FF and Chrome plugins that can display graphs of the price over time of products on amazon pages. Also can email alert you when price drops.
How is this difference than <a href="http://www.pricepinx.com" rel="nofollow">http://www.pricepinx.com</a>? I currently use this site and it tracks more stores than just Amazon.
Just an FYI: <a href="http://www.gazaro.com/" rel="nofollow">http://www.gazaro.com/</a> does something similar to this, and gives users scores on how good the deal is.
I'm a huge fan of HN and with the constant reminders here of people successfully doing something, I've (re)launched an app that tracks amazon price drops. Thanks hackers! Also special thank you to Joshua McKenty of <a href="http://www.piston.cc" rel="nofollow">http://www.piston.cc</a> who built the first version of this.