If you already have a Gmail account you should download Google Talk.
I had downloaded and tried it sometime back. I was not happy with the quality and removed it from my computer as I hardly used it. Only installed it today on my sis-in-law's recommendation. Her office doesn't allow Skype and that's the only application she can use to chat with us.
Was pleasantly surprised at the sound quality - it's great. The only drawback is that there's no video.
It one of the great software in the Google Pack. Just click the recommended download on the sidebar to the right for more info.