HN user
Great list, just to save him and others some time and pain, I evaluated all of those in the list some months ago, and my conclusion is Jitsi is the only one multiplatform, encrypted voice, still under development, free open source and not unusably buggy. Or technical - grandfather has no idea what a NAT, ICE or STUN is.
ColinWright's question is about computer-to-computer calls.
His question is similar to one I've asked recently: Skype replacement for phone-to-computer and computer-to-phone (Skype-in and Skype-out numbers).
Can you please chip in if you know more? https://news.ycombinator.com/item?id=6921672
Can't tell about WebRTC or browser-based. When it comes to software, I've tried everything from the Wikipedia list and some more.
Most are either too buggy (Linphone) or closed source (many) or not multiplatform (so you can't talk to WindowsXP-user grandfather or vice-versa) or development stopped (Twinkle) or has no encryption (Ekiga).
The only one who passed was Jitsi.
It has more than 10 years of development, it's free / open source, very usable, multiplatform. It also accepts encrypted voice over XMPP, not only over SIP.
Look here - https://jitsi.org/Main/Features