I recently starting using camelcamelcamel.com to make me a better informed Amazon shopper. It's main feature is the ability to generate alerts based on price and availability changes. To track a product, you set a price threshold at and below which an alert is generated, assuming the product is also in stock. In this context, alerts can be sent to you via email, Twitter, or RSS feed, containing current price information and useful links.
Another great feature is the ability to track a product's price history so you can tell just how low Amazon has gone on the price in the past and also when that happens so you can keep an eye out during that same time period the next week, month or year.