I'm sure that at least once you heard about Skype, but maybe you didn't install it to see how it works. Well, Skype is great if you want to talk with your friends, because at the video and audio conversation the quality is brilliant. Also, the file sharing is very useful, because they're sending quickly and the size limit is the biggest possible, even 5 or 6 times bigger than at Y!Messenger.
A very promoted feature of Skype is the one with you can dial or send text messages to USA or Canada, from anywhere in the world, for free or from USA or Canada to anywhere in the world, also for free! But the wonder doesn't stops here! You can dial and send text messages PC-to-phone or phone-to-phone via Skype from anywhere to anywhere by paying them at a very low price. I forgot to say that everything that's PC-to-PC is absolutely free!
The newest feature of Skype is also the most useful and here I'm talking about...
the integration with Facebook, which actually means that if you're online on Skype the Fb Updates will appear in your homepage and you'll can talk with your Fb friends via Skype, actually Facebook-to-Skype conversations!! Isn't it gorgeous?!?
And about the smartphone's application there's no problem! For every OS there is an official Skype app.