I said Skype was free, and it is for the most part. To be able to use more advanced features you have to pay, but the basic chat service is free.
Skype is basically an instant messenger for voice. Most IM clients support voice, but the quality of Skype is great! Just download skype, create an account, and add friends. It's simple.
You need a mike to use skype for what its intended. Any computer mike will do, but for really good sound quality invest in a nice headset. I bought the Logitech Internet Chat Headset and I Love it. Its comfortable, sounds great and its reasonably priced.
One thing Skype leaves out of the free version is an answering machine. This can easily be fixed by downloading SAM. Sam is a free, fully functional answering machine program that everyone who uses Skype should have.
Skype works on Mac and PC so don't worry about not cross platform compatibility.
Get Skype For Windows
Get Skype For Mac
Get Skype For Linux
Get Skype For PocketPC
Let me know if you get Skype set up, or are already using it.