There are several options, and in fact, wpcentral has a few articles reviewing the major contenders. Try searching through the last week or two of reviews. I believe the apps they reviewed were:
Turn by Turn
GPS Voice Navigation
Garmin Street Pilot ($$$)
Someone correct me if I'm wrong but if you can skate by for a few months, or with one of the basic cheap GPS apps, Mango should have integrated voice turn by turn navigation with it.