Are you tired of big companies lining up to take on Square? Well, the list officially got longer today with Groupon's (s grpn) launch of Groupon Payments.

This shouldn't be a surprise --  we previously reported that Groupon was testing out the service. But it adds yet another competitor to a very crowded market. Groupon is launching with an iPhone and iPod (s aapl) touch reader that works with the Groupon Merchants mobile application, allowing businesses to take credit card payments.