Jitsi also uses ZRTP for audio and video on the desktop, and there's RedPhone on Android and Silent Eyes (Silent Circle) for both Android and iOS. There are probably a few others, too.

I'm not sure if it can be used with WebRTC clients, or if it's even needed there.

Personally, I'd stay away from anything that is closed-source (as we don't know what's happening inside) and/or centralized (because it's just one more attack vector and totally unnecessary for VoIP).

Which of the above don't involve a central server? A server is absolutely necessary for VoIP, if only to set up calls (its also the only way to deal with some NATs).

RetroShare is completely decentralized/p2p/OTR.

EDIT: @JshWright - Not sure I understand your question, but RS does "real-time voice communication" (besides the other things like chat, messaging, file sharing, forums, channels...)

Isn't the article about a real-time voice communication?

That's Retroshare. It's also the only one as far as I know that has P2P "e-mail", too (besides Bitmessage, which is still quite new and unproven).

BitMessage has already proven to be breakable (as usual, by tricking the user, but still). See https://news.ycombinator.com/item?id=6263850.

Twelephone gets round that by using twitter for call set up only & transmits media peer-to-peer using WebRTC.

The issue is that behind certain NATs, WebRTC's peer-to-peer component can't work. In these situations, it needs to fall back to using an ICE/STUN server, or some other form of proxy between peers.

it's actually called "Silent Phone"