We have launched free FAA Approach charts for Android and iOS tablets and phones. As long as the FAA provides access to the charts for free, the app will update all approach charts on a 28-day cycle, in line with the FAA.
The app allows for easy selection of the charts you need, either by entering one or more airports, one or more states, or a route. When entering a route (e.g. 'ORD to JFK') all airports along the direct line between these two airports (along a corridor of about 50 nm) will be downloaded.
Once airborne, you can easily find the airports you're interested in by searching by identifier, airport name or city name, and then select the desired chart. Starring an airport makes it even more easily accessible, and a history tab gives you immediate access to recently consulted airports.
Note that charts must be downloaded BEFORE going flying, but all downloaded charts are accessible when flying.