There's a freeware Nav program that's up and running on the Mac, Linux and Windows. It uses the US Census Bureau TIGER database as its source of map data, which it turns into graphical representations. It has a 3D mode and turn-by-turn directions. No idea how good the directions are and the user interface could use some work. It has both user and developer documentation.
I don't know how well the TIGER database is kept current between censuses but apparently it's just been refreshed.
Thanks to MacMobile for finding the link, which is on his Web site.
Tom tom has released their new GO X10 series. If i am correct, there is some new software included for connection to your Mac. Its called "HOME". I have heard rumors that it is (or will be in the future) possible to use navigation on your computer screen. It is not yet released in Europe, but in the States it is available.
Hmm, a lot of news, mostly not that relevant, but the nRoute might be what we all need :-) I just hope it will not be limited to PowerBook/iBook (like R66 used to be - you could not do the full installation on the desktop computer).