Get It Later is an application, that allows you to monitor price of any product and notifies you, when it drops below your expected price. It is great for stuff like computer games, where prices drop radically about a year after the release.
This extension will add an icon to your browser's toolbar. When you see some product you would like (e.g. at Amazon, Steam, Xbox market, etc.), just click the icon. It will ask you for your e-mail and expected price. Yes, it's that easy. After that, it will find that product on all possible sources and start to monitor it's price for you.