Here's how it works: Copy the URL of any product page, then paste it in the corresponding field on the ShoppingNotes site. Enter your e-mail address, click Get Alerts, and you're done!
As soon as the product goes on sale (i.e., its price drops), you'll get an e-mail notification. It's perfect for those items you're not in a major rush to buy, but want to score on the cheap.
To make this even more convenient, there's a ShoppingNotes bookmarklet you can add to IE or Firefox. Just click it while on a product page and you'll automatically get alerts, no copying and pasting required.
If you end up using the service regularly, you should register for a free account so you don't have to go through the e-mail confirmation process every time you create a new alert. That also enables you to make adjustments to your alerts, let setting price triggers and alert durations.
Needless to say, there are lots of price-alert services like this one. If you've found one you like better, tell me about it in the comments!