We've seen a couple of different smartphone apps aimed at making it easier to book taxis. If you'd prefer to use hire cars -- where you book in advance and agree a fare prior to travelling -- Get Picked Up offers a booking service through its web site or via mobile phone apps.
Once you submit a booking, it is broadcast to any hire car drivers who have registered with the service. Drivers then submit quotes and estimated arrival times, and you can pick the one you want. Once you've done that, the actual driver rings you to confirm the arrangement.
As with the similarly-themed taxi apps, the usefulness of this service will depend on how many drivers are registered in the area where you need to travel. Get Picked Up is a free service, requires registration. There are Android and iOS apps currently available, and a BlackBerry app is in the works.