WCSART uses this app for team communications
Map Tools App
Has converter features for UTM to Lat\Long
Epocrates
Drug interactions and reactions database
Camera App
While not an app you need to get on a smart phone you should be encouraged to take pictures of briefing boards clues, trail signs and even weather for reference later
Compass App
Can be handy when your compass is in your pack
PDF Reader
While this is build into most smart phones it can be used for storing and reading everything from team SOG's and Medical Guides to storing all the topos for the service area. Download the Wolfe Co topo maps in PDF here
CMC App
Excellent field reference guide for the high angle rescuer
Google Earth
Got to see where you are searching... don't you?
Mag Light
This app magnify and takes pictures of everything. Like a magnifying glass