I've found a free and open source video calling/meeting service with secure end-to-end encryption - JITSI
You can use it for one-to-one chats or you can invite several people to join the conversation.
There's no software to download or install - it simply runs in a browser - and no account or other form of registration is required.
It's easy to use and seems to work pretty well (I tested it in Opera, Chrome and Firefox on Windows).
You can start a call or meeting here: meet.jit.si
Type a name for your meeting and click the Go button.
It'll start pretty quickly and once it has you can copy the link out of your browser's address bar and send it to others, or click the 'I' icon at the bottom right of the screen to add a password.