I use the free memory map app, which is only meant to be used to test if the maps you have will work with the app, but it's a good back up if everything else fails.
I also have a good GPS tracking app, it was brilliant during my IR as it gives a 3D track at the end of your flight, so you can see how good your holds were and how close you came to certain beacons when overlaid on google earth.
I haven't found a decent weather app that works nearly as well as the iphone equivalent, so I just use the browser with a bookmark to the met office, and raintoday for the rainfall.
The phone in general is the best back up to have in an aircraft. GPS, 2 way communication with the ground, weather updates and even a back up torch if all else fails at night.