4. WhatsApp Messenger
There are tons of free and cheap messaging apps available for the iPhone, but there is a reason why WhatsApp Messenger has been a steady presence in the iTunes top 10 paid apps chart for so long. It simply does what it says on the tin, enabling you to freely message your mates and family members that also have the app downloaded onto their iPhone, Android, BlackBerry or Nokia phones. Saves text-fiends a fortune!