Groupon is a deal-of-the-day website that is localized for major geographic markets around the world. The deals generally range from 50 to 90 percent off on dining, events, attractions, and more.
The way the site works is it offers one "Groupon" per day in each of the markets it serves. If a certain number of people sign up for the offer, then the deal becomes available to all.
For more information check out their web site.