The smartphone has become a very useful tool - even for Amateur Radio.
Here are some of my favourite apps:
Easy QTH Locator
Activate GPS on your cellular, and you will see the coordinates and the current QTH Locator.
EchoLink
From time to time it is the only chance to communicate with other Amateur Radio OPs.
IZ2UUF Morse Koch CW
Incredibly useful for training CW.
QRZDroid
Just type in the callsign and you will see the informations on qrz.com.
RepeaterBook
The World's Free Repeater Directory App.