Fans of online shopping will love Product Observer, a Firefox add-on that monitors the price of your favorite products and alerts you when a bargain is in sight.
First things first. Product Observer supports Amazon (.com/.ca/.co.uk/.fr/.co.jp/.de), BestBuy.com, Newegg.com, Zappos.com, Borders.com, Overstock.com and Argos.co.uk. While this isn't a huge selection of shopping sites, it does cover many online shopping needs, and integration with Amazon is particularly well-developed.
When you install Product Observer, it will take you though an initial set-up process, where you can import an Amazon Wish List if you want. You will the see a blue button appear on the product pages of the websites mentioned above. If you click it, you'll be taken to a Product Observer set-up page where you can specify what elements of the product you would like to monitor.
The elements available depend on the website in question, but they range from availability and price to the number of product reviews added (on Amazon, Bestbuy and Argos). In tests, the blue Product Observer button worked on all the websites with the exception of Overstock.com and Argos.co.uk, but this was probably due to our location.
If you are a regular user of the websites supported by Product Observer, it's a great addition. When your product meets the requirements you specify when setting up the monitoring, it will alert you, and you can access your monitoring account on other computers - although you seem to have to install the add-on anyway.
Unfortunately, certain changes mean that this add-on is no longer available.