I think Waze is my favorite app. It's a GPS-based navigational app which uses turn-by-turn navigation, as well as user-submitted travel times and route details, and learns from users' driving times to provide routing and real-time traffic updates. It's free, and now owned by Google. Additionally, people can report accidents, traffic jams, speed traps, police and can update roads, landmarks, house numbers, etc. Waze also helps users find the cheapest, closest gas station around them or along their route.