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...

Continuing on my best-of collection of freeware (although not open-source in this particular case) outperforming their professional counterpart, here is one that, for a good part, made me come back from the Windows Mobile to the Sony Ericsson world: myPhoneExplorer

I started around 2005 with a very ambitious phone manager called floAt's Mobile Agent (aka FMA), but its development suddenly stopped around 2007 while myPhoneExplorer was already becoming increasingly popular.

Much lighter to install than the official Sony Ericsson PC Suite, myPhoneExplorer actually provides a lot more functionalities.

It gives the total control over any Sony Ericsson phone: contacts, calendar and notes can be synchronized with either PhoneExplorer only, Google, Thunderbird, Outlook, and any iCal compliant source. Additionally, photos can be synchronized too, and more recently (since v.1.7), any files on the phone, which comes handy, if like me, you use the phone as a flash drive to bring back home files downloaded elsewhere. The Phone keypad function allows to control the phone navigation from the computer as well as taking screen shots from within any phone application.

The developers are very active and updates are constently published with new and improved functionalities.

After sitting on it for two weeks, the Irish customs finally decided to release the G705 that I ordered on eBay and almost considered lost. The OS was predominantly in Chinese, so I had to change the CDA with tools called "A2 CDA File Generator" and "A2 uploader", so my next update on SEUS would load an European firmware. Then I could really start playing with it:

(I initially posted a review on www.esato.com which forum is excellent for everything related to mobile phones)

I found a good balance between simplicity of use and technological integration in my previous Sony Ericsson phones K750i and K550 and they were part of my organized life. All it needed was a good PC companion freeware, myPhoneExplorer in my case, to synchronize with Google Calendar and backup the phonebook.
All these became suddenly more complicated with the arrival of the HCT TyTNII. The blame is not on the device itself, which is all a geek can dream of, but to the Operating System (Windows Mobile 6.1).
I manage to force this device into my organization habits but it required some additional software:
- Softick Card Export II: to use the TyTNII memory card as a smart drive on PC, as I used to do naturally with my SE phones.
- BirdieSync: to force ActiveSync to synchronize with Thunderbird instead of MS-Outlook.
(Don't get me wrong, I'm not an absolute MS phobic, I just like to have the choice.)
These two applications, despite they are no freeware, are the main reason why I kept my TyTNII for almost a year.

With the arrival of the G705, I could resume on using myPhoneExplorer (which I really missed) and retrieve the legendary simplicity of use of Sony Ericsson phones.

The only regrettable evolution is that now the use of MTP protocol when plugged in Phone Mode device on the computer. The Mass Storage mode used before was more flexible for synchronizing non-multimedia files. Hopefully the USB Ethernet Emulation driver allows to reach the phone's content as a network drive through \\g705... but it's slower.

In both TyTNII and G705 cases, I consider that such a small gadget cannot be your main in-car GPS device. For this there are cheap GPS with large screens and fast chipset (such as SirfStarIII) out there that do the trick much better.