With "In-Network" calling you can call between VoIP accounts for free, with most any VoIP service.

All Skype,Yahoo Messenger, and Google Talk users can talk to each other for free over Internet as long as they each have accounts with the same VSP (VoIP Service provider). I talk to my brother all the time over Skype or Google Talk PC-PC for free.

But, if you want to use a VoIP Phone using an ATA (analog telephone adapter) to talk to other VoIP phone or PC based Softphone users, then you can use services like CallCentric and InPhonex.

With these services you can talk to any other user on their network for free (In-Network calling). You can call from PC-to-ATA, ATA-to-ATA, or ATA-to-Softphone for free. And of course to PSTN landlines as well for 1.98¢/min with CallCentric.

I use my own user configurable (unlocked) Linksys ATA (PAP2T-NA or SPA2102-NA). I use CallCentric on the ATA line-1, and InPhonex on Line-2. Two separate services, two phone numbers, but only one VoIP ATA box.
CallCentric and InPhonex both have free downloadable Softphones. However, they are BYOD (bring your own device), which means you have to buy and configure your own VoIP ATA phone adapter, like I did.