Shazam, Skype and Ebuddy are very popular (free) apps for your iPhone. And if you are on facebook, check out the facebook iPhone app (this is also a free iPhone app)!
And if more of your friends have an iPhone, check out this (almost free) app that allows you to send instantly messages (SMS) completely free: Ping(http://www.pingmessaging.com/Ping!/Ping!.html)
Again, only works if your friends also have an iPhone, but you can really save costs here and it's easy too!
I'm curious to see which other free iPhone apps will show up over here?