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.

To make a call, just click the Call phone link in Gmail and enter any number or name from your address book.
All calls made from Gmail will display your Google Voice phone number as the outbound caller ID and all international calls will use your Google Voice calling credit and are offered at the same low Google Voice rates. We took great care to make sure that our rates are as low as possible. For those of you not as familiar with international calling rates, check out our comparison table.
Finally, check out this video:
We’re rolling out this feature to U.S. based Gmail users over the next few days, so you’ll be ready to get started once “Call Phones” shows up in your chat list (you will need to install the voice and video plug-in if you haven’t already). If you’re using Google Apps for your school or business, then you won’t see it quite yet. We’re working on making this available more broadly - so stay tuned!
For more information, visit gmail.com/call.
Update (8/26): This has now been rolled out to everyone in the U.S. If you don't see the feature yet, try logging out of Gmail and signing back in.