These are the main apps I use:

WORK RELATED

PilotsGuide (Airport, weather etc)
FlightWare (Flight tracking)
AeroWeather (Precise weather forecast)
FlybyNav (Flight planning)
Viber (WiFi calls and texts - although the person on the other end must have this app as well)
Gorilla Expense (our expense management application for aviation departments)
Wi-Fi Finder (finds nearby Wi-Fi locations)

FUN

SoundHound (Recognizes any song, music etc and gives you the name)
WeatherChannel
Paper Toss (addictive game)
SkyView (constellations, space etc)

I am finding that using too many apps actually lowers my efficiency.