Several Sony Ericsson GPS-enabled mobile phones come with Wayfinder as the main application. The software itself looks ok and could even (very) occasionally be used to guide you when driving. But there's a catch: after a trial period, you'll be invited to pay a yearly fee. And instead of availing for their "exclusive offer", you may prefer to buy an entry-level but decent dedicated GPS device for the same price!
However, there are free alternatives if you still wish to use your mobile phone as a PNA:
amAze is a free GPS application (ad-funded) working on most J2ME mobile phones.
An important trick to avoid expansive data traffic for streaming maps over the phone, is to use the arrows mode where only the small amount of data for the calculated route is sent to the phone. See more here.
Like Wayfinder, it also provides vocal instructions. Previous reviews of amAze highlighted the ugly interface and the slow response. All this has visibly improved now (I tried v4.5). It actually seems slightly ahead of its competitor: Nav4All . But this is open to discussion...