iTranslate - very handy when you are in the supermarket!
Skype - i use this while I'm at home to call UK landlines for free.
Facebook - not great when you want to type lots of messages but good for catching up on all the gossip.
SBB Mobile - this is great for quickly finding train and bus times.
WhatsApp - i think this is less than 1CHF to buy, and it gives you free text messaging to anyone else who has an iphone.
Phonebook