The app is called Skobbler US. It works on the iPhone 3G and the 3GS. The map data comes from OpenStreetMap, a sort of Wikipedia for maps.
Skobbler features turn-by-turn GPS navigation with voice assist, iPod control, a night mode that is automatic or manual, pedestrian navigation and a 'take me home' button.
It all sounds good until you use it. First, it's really slow, both in start up, and in displaying your position. It often showed me at an intersection about 15 seconds after I passed it. The routing information was pretty brain dead. I just didn't find it giving me good advice on the routes I selected. It was also slow to re-route me after I decided to ignore the bad advice I was given.
The iPod integration was a C-. It did see my music library, it did show the tracks. When I selected podcasts, the list was empty even though there were 4 podcasts available.
This app requires an internet connection. The maps aren't pre-loaded. That keeps the app small, only a 2.3 MB download.
I think the biggest drawback is the way it operates. If you just want a map of where you are you can't get one. When Skobbler starts up it wants a destination address. If you don't input one, no map. There is no "points of interest" database, so if you are looking for the nearest restaurant, gas station or hospital you can forget it. Google shouldn't be too hard to integrate into Skobbler, and I think the developers should get on it. As a result, the only way to use this app is to specify an address. That really does limit how you can use it. I tend to do a lot of proximity searches, and Skobbler just doesn't do that.
The app only displays in portrait mode, and I really prefer landscape views. The street maps were better than I expected. It seemed to show accurately a lot of small residential streets here in Arizona, and it was able to navigate to my house which some of the other free apps can't do.
Skobbler is free, and I can't get too down on it, but I just don't think it's going to be a viable solution for those expecting a full or even reasonably complete boatload of features.
Most of the commercial nav apps are dropping prices. Both Navigon and TomTom are running sales this summer. Skobbler is a nice try, and may be enough for some, but I doubt it is enough for most drivers.