I've used Google Voice for a long time, as a way to protect the privacy of my actual home phone #. I love it. It can be used as a stand-alone voice-mail service, or it can be simply forwarded to one or more phones. But it just got even better.
Now, you can make calls from your computer using Google Voice - for free within the U.S. and Canada (rates apply to international calls). It's easy. You'll need a Google account, of course (does anyone seriously not have one of those? Come on! If you use G-Mail, when you're logged in, in the lower left column, you'll see a "Call Phone" link with a little phone receiver. Click on that, and follow the instructions. Or, just go to voice.google.com, and sign up from there.
I tried it - the sound quality is great, and it's simple to use. Go try it!
Also see: Mashable: Google Brings Voice Calling to Gmail and TechCrunch: Google Voice Integrated into Gmail. Make and Receive Calls from the Browser.
Jennifer Pointer (e-mail) is in Tulsa, OK. She promotes a simple, a low-tech approach to effective online profile management, search engine optimization and social networking.