Firefox only (Windows/Mac/Linux): Firefox extension BuyLater adds a Buy This Later button to Amazon pages that sets up a watch for the product on the BuyLater web site. The web site then checks Amazon for updates every minute, alerting you by email or Twitter of price changes and availability. I've just set up my first watch, but if it works as advertised BuyLater is a perfect tool for tracking often out-of-stock items—like the Wii or Kindle—or just for watching for price drops. The quick notification system, in theory, will ensure that you'll know as soon as the price changes or item becomes available, giving you first dibs on the product. BuyLater is free, works wherever Firefox does.