This is built in to the Maps functionality on the iPhone. Typing in "taxi" will show me the details of the nearest cab companies and touching the phone number t

This is built in to the Maps functionality on the iPhone. Typing in "taxi" will show me the details of the nearest cab companies and touching the phone number that appears will let me call them. Getting the nearest free cab to drive to me would take a little bit more than an iPhone app - you'd need a whole new business model built around it.