Turn-by-turn navigation has come to the iPhone via xGPS. You'll of course have to jailbreak your iPhone for this, but this might be the one app that might entice you to risk bricking your iPhone - or at least drawing the wrath of Steve Jobs. It actually leverages downloaded Google Maps for the GPS coordinates and just recently added a voice engine to "speak" the directions. It also apparently works on the iPod touch, though you'll need an external GPS receiver.
What's new in the latest version of xGPS?
Wireless Transfer of your map files. No longer do you have to dig around your iPhones directory to find your map file or put it in the right place. Simply download your maps from your computer and wirelessly push them to the xGPS program on your iPhone. It's that easy!
Google Terrain Maps are now supported and added as another option for maps. All your hikers, bikers, and adventurers can enjoy this!
GPS Information. Access your GPS's Lat/Long specs as well as Altitude and Speed of your unit and sent it by email.
English Voice instructions are now supported. Now included is the ability to receive verbal commands on your next turn as you approach it.
Night Mode for all you road warriors traveling across the country all all hours of the night! A new, easy view for the eyes when driving at night!
Access your GPX traces on your computer using a normal web-browser using the Wireless Transfer feature.