Waze is a brilliant and free (we like free!) satnav app for Apple and Android.
Imagine millions of drivers out on the roads, working together towards a common goal: to outsmart traffic and give everyone the best route to work and back, every day. You just hit a button to notify everyone around you about any traffic problems. This means you get alerted before you approach police, accidents, road hazards or traffic jams, all shared by other drivers in real-time. It's like a personal heads-up from a few million of your friends on the road.
It re-routes you if there is a better route and, to be honest, I find it more accurate than any of the paid ones. I even used it to drive in Thailand and managed not to get lost! (It even showed us the missing bits of road that you often find on Thai roads!)