Lifehacker came up with a good way to get free Google Voice calls on your desktop with Sipgate, but Sipgate seems to have run out of free numbers, rending that method broken (for now). VoxOx can do something similar.
Here's the deal: When you sign up for VoxOx you get 1 free number that you can use to hook in to your Google Voice with. You also get free incoming calls (which means incoming AND outgoing calls with Google Voice, cause GV always is the one to call your phone), incoming texts and incoming faxes. Outgoing calls (if you're dialing straight from the VoxOx client), outgoing texts and outgoing faxes cost money. So while you can get incoming texts to your VoxOx account, responding using that IM window is a bad idea, since you get charged about a penny per text.
I tested the call quality and it wasn't bad, but it wasn't great either. It sort of varies between "yeah, good enough since you're not paying for it", to "man, I really have a hard time hearing you." But it's free, which is the main draw, and if you don't have a landline at home and you don't want to use up your cell minutes, it's kind of a great way to make/receive calls. Voice lag—already a problem with Google Voice—doesn't seem to be exacerbated too much with VoxOx, but it doesn't help, and it's not as instantaneous as it is when you're Google Voicing with a landline.
But until Google Voice's desktop client that supports incoming/outgoing SIP-based VOIP calls gets released, this is a decent enough alternative, even if the UI design of the desktop client is a very late '90s red-on-black affair. Also, VoxOx tells us that they'll never run out of numbers, like what happened with Sipgate. [VoxOx]