Yesterday (2009-11-12), Google acquired Gizmo5. Downsides: The bit about configuring a Google Voice number below is no longer possible (this configuration part is completely gone), and more importantly, Google has closed Gizmo5 account sign-ups for now.
Read more at Google's Gizmo5 page at http://www.google.com/gizmo5/, and you can read the official Google Voice blog post at http://googlevoiceblog.blogspot.com/2009/11/google-welcomes-gizmo5.html.
Original blog post, unmodified, is as follows:
Well, I hate paying for phone service, as does everyone else. For the longest time, my only phone was a shitty TracFone, and for $20/month, I'd get a whopping 60 minutes of talk time.
How fucking impressive.
The worst part was that SMS is also expensive, costing 0.3 minutes of talk time per message sent or received, coming to about 3 minutes of talk time for every 10 messages in and out.
Until I found Google Voice.
Google Voice was originally something I used to replace the SMS on my phone, as you get unlimited and free SMS to any phone in the US and Canada. However, this did not solve the phone issue, as Google Voice requires a real phone to use the service.
Well, one day, I got bored, and decided to see if I could use my Gizmo SIP number with Google Voice. To my utter amazement, it works, and since then, I have had nothing but free, FREE, unlimited calling from and to any landline in the US and Canada, using a simple VoIP app.
You want to do this? You want to tell your phone company to take a hike?
There's one thing that's not mentioned in that article. Read that article before you continue below.
Read the article? Good. Now, you may notice that it says that to PLACE calls, you have to use the Google Voice web page, and have Google Voice call your Gizmo.
Not anymore. Gizmo5 now supports calling though Google Voice in the same free manner, directly from Gizmo5. You place a call from within Gizmo5 as if you had paid for Gizmo5 call-out credit.
But to do this, you need to change some account settings.
Login to your Gizmo account via the Gizmo website, and pull up your account settings.
There should be a new option there, labeled "Google Voice", and should look like this:
Click the "ON" radio button, type in your Google Voice number, and that's it! You no longer need to go to the Google Voice site to call out using Google Voice.