Tried using the Navigon navigation software to get home last night and although it works, it feels a bit basic, so I'm trying iGo and Sygic next, you can have what ever suits you though. The best one and even P's on TomTom etc is the Google navigator, it has traffic updates, actual pictures for turns and satellite imagery!
Have a look here, the only down side is that you'll need a web connection to use it, although I was listening to Di.fm for about an hour of my journey yesterday with only a couple of interruptions.
http://www.google.com/mobile/navigation/