Google uses open protocols and it's very possible to communicate with somebody on a Gtalk account without having a Gtalk account yourself.
Of course you need a account with _somebody_. You can set up your own domain and servers of course. Which is what I did and works fine with people using Gtalk.
The biggest problem you run into is that all phone conferencing software on Linux is very terrible except linphone (I especially like the command line client) and I don't think that supports video conferencing.
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