So far there has been a lack of a good turn-by-turn navigation software for the Windows Phone 7 platform. So to address this issue, Garmin has stepped in and released their StreetPilot app on the Windows Marketplace.
The app costs a hefty $39.99 but comes with a host of features such as the aforementioned voice prompted, turn-by-turn navigation, real time traffic updates, integrated local search, lane assist with junction view, current weather with forecast, etc.
Unfortunately, the application does not come with the maps built-in and they are downloaded on an on-demand basis. But you can download the maps for your route in advance before heading out and if required the application will download the rest over 3G, Wi-Fi or EDGE.