Google also announced today that calls to the US and Canada would be free for "at least" the rest of the year, leaving the door open to possibly charging for calls beginning in 2011. Calls to other countries will always be very cheap. For instance, I used Google Voice this morning to call Ireland for 2 cents (U.S.) per minute. The connection was crystal clear, better than many other overseas calls I have made in the past.
Google Voice only uses your computer to initiate the call. The audio never travels through your computer. You talk on your regular landline or cell phone. You don't wear a headset. In fact, once the call is initiated, you could even turn your computer off and yet continue to talk.
You must sign up for the service and create a Google Voice account in advance before placing any calls. The service is only available to US users for now, and, in typical Google fashion, it's rolling out slowly; don't panic if you don't see the update right away. Some users will see it now while others may not see it for a few days.
Details may be found at http://googleblog.blogspot.com/2010/08/call-phones-from-gmail.html