whatsapp
While there are many different messenging apps out there for the iPhone, of the ones I've tested I have to say I find Whatsapp to be the best by far.  It's very quick, very simple to use and has lots of nice features.  More importantly, it seems like virtually everyone with an iPhone is using it now, for business and personal use alike.

Using Whatsapp, you can quickly text your friends, share photos and contacts with them, or even send them your exact location!  This "location" actually interfaces with the Maps app on your iPhone so if you someone sends you their location and you tap it, the Maps app opens showing you their location as well as your location.  Now that, for me, is a really nifty feature that I've used many times.

It also integrates directly with your contacts on your phone, showing you who's on Whatsapp already and allowing you to invite those that aren't.

Of course, like most messenging apps, you need internet access to use it - but most of us do have internet on our phones these days.  The app costs only €0.79 and is worth every single penny in my opinion!