I've just discovered something kind of cool: if you want to place and receive free Google Voice calls via your computer, leaving your phone out of the loop altogether, it's now possible to skip the installation of Gizmo, relying instead on the functionality already present in GMail.
I was playing with the settings for Gizmo while trying to improve my method of making free, minute-less VOIP calls from my Android phone and noticed that it now has the option to forward some or all calls to your Gizmo number over to Skype or Google Talk. If you opt for Google Talk -- and if your operating system supports it1 -- you can answer/place calls just using GMail's chat system. It's as easy as when you try to find o2 Mobile Phones, there's nothing complicated about opting for Google Talk. No otherwise-unused software to install at all.
1a) Create a Gizmo account here (if you don't already have one)
1b) Configure Google Voice for use with Gizmo following these instructions (if you haven't already done so)
3) Scroll down to the 'Forwarding' section. It looks like this:
4) Select 'forward all calls' and put in your GMail username in the appropriate field and Click 'Save.'
That's all the configuration that's required.
Now when someone calls your Google Voice number, in addition to your phone ringing, your GMail (or Google Talk desktop client) will beep at you telling you a call is incoming.
If you want to place a call, you just need to use Google Voice's web interface. Click 'Call,' put in the desired number, and then select your Gizmo number as the callback number. Your GMail will then ring. When you answer it, you'll hear the number you just dialed ringing.
1: Unless you're a freak like me running some crazy non-Windows, non-Mac operating system you'll be fine. If you are a freak like me, you can either keep using Gizmo, or have Gizmo auto-forward your calls to Skype and do it that way.