There are two ways to use ShoppingNotes. First, you can copy and paste a product-page URL into the site's Web form, then enter your e-mail address for receiving alerts. Alternately, you can install the ShoppingNotes bookmarklet, which greatly simplifies the price-watch process: when viewing a product page, just click the bookmark.
You can also sign up for a free ShoppingNotes account, which lets you view and organize your alerts, get up-to-date prices, and so on. Definitely a handy little service for anyone waiting for a bargain to come along. Of course, there are loads of other price-watch services--this one just happened to cross my radar. If you've had good luck with another service, tell me about it in comments!