There's nothing like having a good set of topographic maps when you are hiking in the back country. Paper maps are the de facto standard, but they tear easily, get wet in the rain and can easily blow away with a stiff gust of the wind. If you have an iPhone in your pocket everywhere you go and want a more permanent set of maps, then you should check out Topo Maps+ from Glacier Peaks Studio.
As someone who spends a lot of time outdoors, I can reliably say that the most important feature of Topo Maps+ is its support for offline usage. You can be deep in the woods, far from any cell signal and still use the app to navigate from waypoint to waypoint. Before you hit the trail, just remember to download the maps in advance. You can then use the iPhone's GPS and compass along with the maps in the field.
For planning purposes, there's also a trace route feature that allows you to layout your trip by tracing your path on the map.The traces can be superimposed on a topo map and then viewed on a satellite map, allowing you to get a different view of your path. You can also use these traces to estimate distances and elevations between two waypoints.
Topo Maps+ is available for free in the iOS App Store. There is a demo mode that allows you check out the major features of the app, but you will need to buy a subscription if you want usable maps in your location. Pricing starts at US$4.99 for 2 months of access to all the maps or 6 months of access to only the USGS & Thunderforest Topo Maps. Year round access to all the maps will cost you $14.99.
Topo Maps+ include the following maps: USGS 7.5 minute topo maps for the continental USA, Thunderforest Topo maps, MapBox Terrain, MapBox Satellite and Open Street Map maps.