Kik Messenger's a no-frills app, but it could easily become your favorite way of keeping in touch. It's a simple cross-platform instant messenger which'll work for you and your friends—no matter which mobile platform or country you favor.
Think of the app as being like BlackBerry Messenger without the mobile operating system limitations.
You'll get push notifications and little S, D, and R indicators showing if a message has been sent, delivered, or read. Simple and basic, but pretty great.
The only genuine annoyance I've encountered with Kik is the lack of an option to turn off message previews on the lock screen. Don't get me wrong, I like knowing who messaged me, but I don't always want the content of the message displayed on the lock screen. But considering the stability and simple awesomeness of the app in general, that's an easily overlooked flaw for now.