Are you looking for apps that merely assist you in performing a specific function while in the field? If so:
DSLR Dashboard
Sun Surveyor
PHOforPHO
Photo Tools Pro
The Photographer's Ephimeris
Sun Trajectory
Moon Trajectory
Moon 3D
Eclipse
A whole bunch of Audobon guides
DOF calculators
ND Filter calculator
Stopwatch & Timer (great for exceedingly long exposures!)
Various GPS apps, both for my location as well as geotagging
If your looking for apps that teach, I'd suggest books instead. You really can't learn out in the field while trying to read an app.