I was recently introduced to Über by a friend and it truly is a fantastic service. Basically you can use an app on your ipad or Android phone to find high-end taxis near you in large cities in the US and Europe. Your credit card is on file and payment is made electronically. They even send you a message with your receipt and trip route. In most cases Über is priced competitively with taxis but check the rate card on their website. Recently I was on a trip to Europe and we were in a not so great part of Paris, it was raining and there were no taxis in sight. I tether my iPad from my friend's iPhone and pushed 2 buttons on the Über app and we had a Mercedes in 10 minutes. Über is awesome!