Coaster lets you order and pay for drinks from your phone. With just a few taps, you can send an order. The bartender makes your drinks & sends you a notification letting you know your drinks are ready. Show the bartender your order ID and voila, you have your drink. Payment is completed through your Coaster profile and tipping is included.
iOS: http://itunes.apple.com/us/app/id478256206 Android: https://play.google.com/store/apps/details?id=com.coasterapp.coaster