GoRadar on the iOS App Store only, is a free app which is more accurate than Pokevision! Like Pokevision it shows the exact location of the Pokemon around you, but this shows the exact time to the second of how long the Pokemon is available to find and capture!
Edit - After using it for a few days, I've noticed this works far better in busy areas (eg city centres, shopping malls etc)
Edit 2 - Or.... If anyone wants to gain an advantage but doesn't want to jailbreak their iPhone, but can be asked to hook up their iPhone to their PC/mac.... https://m.youtube.com/watch?list=UUj34AOIMl_k1fF7hcBkD_dw&v=CgkoUS8g9C4