Not to get too off topic from the outbound vs inbound discussion, but I often use GV to ring either a SIP app on my iPhone, or a home phone that's connected to a
LinkSys VoIP adapter (that also uses a SIP account).
Several SIP providers offer free incoming calls, and only charge for outgoing. If you use Google Voice and have it ring a SIP number, the SIP provider considers it an incoming call, making it 100% free.
Since these free calls have to be initiated from Google Voice (to make the SIP account think it's an incoming call), I usually use my phone to initiate it (instead of walking across the house to my computer). For example, I use VC to start a call and have it ring the SIP account that the iPhone app uses. VC sends the info to GV, GV sends it to the SIP provider, and the SIP app shows a push notification that says I have an incoming call. I answer, and it's free! (If you have a limited data plan, that would need to be considered, but I usually only do this on WiFI anyway). Basically, it's:
VoiceCentral -> Google Voice -> SIP provider (seen as a free incoming call) -> Acrobits Softphone (iPhone SIP app) gives push notification -> free outgoing call!
Sounds convoluted, and it is - but once you get everything setup, it works fairly well. I've helped a few friends do the same thing on their iPod's - if they have headphones with a mic, they can also make free calls directly on the iPod.
I'm sure most people don't use it this way (it's complicated and has occasional issues), but it's one of the more robust ways you can take advantage of Google Voice/VoiceCentral.