Who doesn’t like a deal? Everyone does. And Groupon has mastered the art of the daily deal. You sign up for Groupon, and soon you’re getting deals for books ($20 of titles for just $10), spa treatments, cheeseburgers, water-park adventures, and just about anything else you’re able to do or buy. Print a coupon (or present it on your mobile device), and soon you’re saving big bucks.