Or Jitsi:

https://jitsi.org

I figure P2P encrypted WebRTC calls should work soon, too.

Jitsi does use Jingle with ZRTP :)

Do you know of any non-Java (ideally GTK+) ZRTP+Jingle clients?

Could you point me to any software packages that use XMPP/Jingle with ZRTP?

Also, do you know how the key distribution is managed with ZRTP?

Unfortunately I don't know any other clients besides Jitsi, that work with ZRTP (it doesn't mean there aren't any, I just didn't find them).

Pidgin/libpurple don't support ZRTP yet, as well as Telepathy, which means all major clients based on them don't support it either (such as KDE Telepathy or Empathy for example).

There are long standing bugs about it, but nothing seems to be moving:

https://developer.pidgin.im/ticket/11221

https://bugs.freedesktop.org/show_bug.cgi?id=29904

See also: https://bugs.freedesktop.org/show_bug.cgi?id=16891

About ZRTP itself, see for example: http://my.safaribooksonline.com/book/telephony/9780321437341...

That's very unfortunate. I really wish that stuff like this would take a greater priority.

May be latest concerns about being prism'd by the Big Brother will boost this development.

Or FaceTime