It took me a while to try out Google Talk, but as soon as I did I was instantly hooked. All you need is a microphone and you can talk to anyone you like for free and anywhere in the entire world. This technology has already been developed by companies like Skype and Vonage, but Google puts in a few different features. You can send files directly through Google Talk to the person that you are chatting with. Google Talk also allows you to setup a profile with a pic and you can even leave voice messages to someones e-mail! This service is also integrated with G-mail so you can be notified when a new message is sent to your G-mail account. If you don't feel like talking you can also instant message just as easily.
Yes, like most things Google it is completely free so you have nothing to lose. You can find out more about Google Talk here.