Pick your city and your character, and hit the streets.

Desktop: W/A/S/D or arrows to move & steer, Shift to run (handbrake in cars), E to enter/exit vehicles, Enter/T to chat, C for the overhead view while driving.
Mobile: tilt to steer, tap & hold to accelerate — slide down while holding to brake, up to go — tap near a car to enter.

Open the City Editor → — build your own city and save it as a .gmap.
Edit this city in 3D → — fly the live world and reshape it in place.
Open the Asset Lab → — write a building in Dart, run it, and watch it appear.