I loved "Navigator free 10" on my Windows Mobile 6 (Palm treo 750).
But now, using a "Windows Phone 7.5"-Device, there´s no offline-Navigation except "Navigon Navigator/Select". (Something about 80,- Euros)
A lot of people just waiting for a real good offline Navigation-App, even if it would be a paid-version of about 5-10 Euro. => This could be your chance to be "Number one" with an offline-Navigation on Windows Phone Market.