While most chat services offer text, voice, and video chat from computer to computer, the new Google service allows for calls from your computer to a regular mobile or landline telephone phone.
The Google Voice Blog article about the service explains more.
Google Voice lets you manage all your phone communications and seamlessly make and receive calls on any of your existing phones. But what if you don’t have your phone with you? Or what if you’re in a place with poor cell phone reception, or you’re travelling internationally and don’t want to incur expensive roaming charges? Wouldn’t it be great if you could use your computer to make or receive calls?
Starting today you can use Gmail to receive or place Google Voice calls.
To get started, check the box next to Google Chat in your list of forwarding phones and the next time someone calls your Google Voice number, Gmail will notify you of an incoming call. You can take the call or even listen in as the caller leaves a message, in a single step right from your computer.
Video. Below is an overview showing the features of Google Voice.