Gizmo5, the Skype-like internet phone service, could already be set up as a Google Voice forwarding number, letting headset callers receive and, through Voice's web site, make calls. Now Gizmo allows for straight-up, convenient dialing through Google Voice numbers.
If you're signed up and registered with Gizmo, you can head to your account settings at Gizmo's main site and provide your Google Voice email and password. Then log into its mobile site or desktop clients for Windows, Mac, or Linux with your 1+10-digit Gizmo5 number and password. Now making outbound calls shouldn't require using any of your Gizmo credit, or heading to Google Voice's site to connect the call—outbound calls from Gizmo setups automatically route through your Google Voice number. That setup can be used for U.S. and/or international calls, depending on your preference.
Google Voice is still in an invite-required semi-closed phase, with invitations rolling out on a first-come, first-served basis. Gizmo5, however, is free to use and sign up for. Thanks Andrew!
Update: Andrew points us to a visual Gizmo5/Google Voice setup guide that details the nitty-gritty of getting going with free Giz/Google calls.