Shopping Gets Even More Social Bargain hunting meets viral marketing via Groupon.

Groupon transforms the art of the deal into a truly collaborative effort. The premise behind the social commerce site is simple yet ingenious: Each day, Groupon.com presents an irresistible bargain from a local retailer or restaurant, with just one catch--the offer is good only if a predetermined number of consumers take advantage within a 24-hour period. If the user minimum is met, everyone who signed up is charged for their purchase and mailed a link to print their Groupon to be redeemed; if not, the deal is canceled.

"We tell the businesses we work with, 'Make this the best deal you've ever run,'" says founder and CEO Andrew Mason. "We want consumers to feel like if it's offered on Groupon, it's the best deal available."

The Chicago startup also encourages bargains that promote social interaction among users, such as dining opportunities, spa treatments, even extreme sports--a Groupon offering a $100 discount on tandem skydiving jumps was an enormous success for partner Chicagoland Skydiving Center in August. "We sold over 1,600 skydiving packages," Mason says. "In one day, we equaled a quarter of the number of skydiving lessons they sell in an average year."