Seeing Eye GPS is described as 'The Seeing Eye GPS is a fully accessible turn-by-turn GPS iPhone app with all the normal navigation features plus features unique to blind users'. There are four alternatives to Seeing Eye GPS for iPhone and iPad. The best alternative is Ariadne GPS. It's not free, so if you're looking for a free alternative, you could try Seeing Assistant Move or iMove.
Ariadne GPS is a map that talks to you. VoiceOver and gives you access to information that are normally unavailable to people using the speech synthesizer and can be therefore especially useful to blind and partially sighted people.