Woot

Woot is a website that sells one product at a time.  While this may sound silly, that one product is usually hugely marked-down, and as such, Woot has become a favorite destination for online bargain-hunters.  They usually only change their product once a day, but occasionally, they offer what's called a Woot-Off.  During a Woot-Off, the offered item is replaced with a new offering as soon as it sells out.  Usually the product changes every 30 minutes or so, due to the limited quantities offered.

notify.me provides a perfect interface for interacting with the Woot-Off.  Instead of repeatedly checking to see if the offered item has switched, subscribing to Woot with notify.me allows me to get an instant-message each time a new product is offered.