Many taxi companies offer apps to book taxis, but the results often aren't any different to what would happen if you used the phone. goCatch identifies nearby taxis via GPS and displays them on a map.
To work its magic, goCatch relies on taxi drivers also running the app. Using the app, you can contact them directly to book a pick-up or put out a general call for the nearest available driver to come and grab you.
Obviously, the usefulness of goCatch will depend on how many drivers are actually running it in an area when you need a taxi. Since it's a free download, though, you haven't got much to lose by trying it -- if there's no taxis in sight on the app, you can always ring and book one in the usual way, and at least you'll have avoided the nuisance of ringing to book a taxi and then seeing one turn around the corner 30 seconds later.
goCatch is a free download for iOS devices.
goCatch [iTunes App Store]