Silver Navigator is a new GPS navigation app in Marketplace than is based on Bing Maps. The software initially only supported directions using an indicator sound and on-screen arrows and written directions.
The software has now received a major feature update, with the addition of Voice-based turn by turn directions.
The app is still not quite as mature as a standard GPS navigation app from TomTom or Garmin, but for $3 its a great start.
Silver Navigator can be found by searching Marketplace.