I use a couple different ones:
Waze is a really cool nav, but it sometimes routes you on some lame paths-make sure the right setting are turned on and you'll be fine.
I also use Skobbler, which is also free, and it works better at routing than Waze, but the maps are outdated.
I honestly will use the regular non-voice map that comes with the iPhone if I'm in an area I'm somewhat familiar with. It works the best, and although I have to glance at it a lot, it is probably the most accurate out of the bunch...for free that is.