Best driving app

I had never heard of Waze before but just looked it up. Looks interesting. It's amazing what ideas for apps people come up with that making such great use of modern technology. One of my friends thought of an idea for an app for driving, I think it as mainly for taxi drivers. It would use GPS to track their location, etc. If you have any ideas, let me know. I'm building a list of ideas for mobile apps to work on for the future.