The three popular sites are Groupon, LivingSocial and SocialBuy.
Before you jump in with both feet, do the math. Look at your margins and see if the deep discounts are worth it.
If you are a new business, do you want to start building your brand as a discounter?
If business is hurting you might be tempted to discount but its likely a short term fix for whats ailing.
There are pros and cons to coupons.
Know why it should be part your marketing mix and not because its the flavor of the day.