I've been using skype for a while now and it has worked pretty well. Sometimes it will randomly say that other people are not logged in when they really are, but logging out and logging back in fixes that. That's the only problem I've ever had. The voice quality is reeeeally good, better than if I called someone from the phone in my room. It has a chat function and you can send files as well which is nice. I bought a skype-in number which is a number that is local to my hometown in the US, so my friends can actually call it from their cell phones and it will go to my computer. It doesn't cost either of us anything, it just uses their cellphone minutes. It also comes with voicemail, which is useful.
Also, strangely enough, I've noticed that my slingbox works much better/has much higher bitrates when I connect to my home computer with skype at the same time. I have no idea why this works, but it does.

