iOS: Google updated Google Maps for iOS yesterday with a handy feature that has been available on Android for a while. Offline map downloading with turn-by-turn directions is rolling out for users over the next couple weeks. The offline maps feature works the exact same as it does on Android. When you search for a location, you get the option to download a map, then you'll download the map alongside turn-by-turn navigation and location search for that area. This feature's rolling out gradually, so you might not have it right now.
Google Maps (Free) [iTunes App Store]