Nokia has both Nokia Maps and Nokia Drive. Nokia Maps is equivalent to Google Maps and will be available for all Windows Phone 7 handsets soon, and while it offers routing and can be used for navigation, it does not offer voice turn by turn directions.
For that there is Nokia Drive, in action in the video above. The app offers full turn by turn GPS navigation, with voices in many languages available. It offers turn both 2D and 3D views and also renders points of interest in 3D on maps.
The voices are loud and clear, always useful for a device used for GPS navigation. Routing is accurate, but I think makes odd routine decisions, for example routing me via the ferry (and an unwelcome 30 min wait) instead of the tunnel across the Thames, which tends to be quicker.
Maps can be downloaded to the device, but the app does not work in flight mode, demanding to be online before launching.
Nokia Drive will be exclusive to the Nokia Lumia 800 and 710, and will be 100% free to those users.