This site is early alpha. You can read more about what that means here
The way the site works is that you first create a login, then can create price requests which are you saying "I'd buy this item if it were only this price or below." The price will periodically be checked at Amazon to see it the price has met your threshold and if it has, you will recieve an email to that effect. You can also view a page of the status of all your requests as well as the current price on each item.
Note that when creating a price request, you can enter either the ASIN of the item at Amazon or just paste the entire URL of the page you are looking at. Either will work. Also note that this site only tracks Amazon.com, not any other e-commerce sites. If you have any feedback or experience problems, let us know at feedback@BuyItAtThatPrice.com
Here is a bookmarklet that you can use as a convenience for tracking items.
Step 1: Drag this BuyItAtThatPrice to your bookmark bar.
Step 2: When on the Amazon page for an item you want to track, click that bookmarklet.
Step 3: You should be taken to the page for tracking a new item with the URL already pre-filled. Enter your price and submit it. Enjoy!