Groupon CrunchBase Profile: "Groupon (www.groupon.com) features a daily deal on the best stuff to do, see, eat, and buy in more than 150 cities around the world. By promising businesses a minimum number of customers, Groupon can offer deals that aren’t available elsewhere.
Groupon brings buyers and sellers together in a fun and collaborative way that offers the consumer an unbeatable deal, and businesses a large number of new customers. To date, it has saved consumers more than $300 million and claims it has generated millions in revenue for the businesses it features."
Also look at several copycats that have popped up: LivingSocial, 8coupons and AOL's Wow.com.
I have personally being using Groupon for months and have found a couple great dinner deals - and one great deal on a haircut!