Every now and then a much needed extension is added to the Joomla family, that enables a whole new range of activities and business models. Jomsocial did it for social networking and now En Masse is doing it for social buying. It's still new but the people behind it are very responsive and ambitious in terms of features and integration with other components.
It looks like this extension will eventually have more features than Groupon itself. Thank's to En Masse, Joomla is now a social buying system also.