So even before MS coming into the picture GNU community emphasized a truly open VOIP solution of their own and according to a major contributor to GNU, David Sugar, they've released the first official 1.0 version of the GNU SIP Witch.
What is SIP?
SIP (session initiation protocol) is the basic foundation of the major VOIP frameworks (although, SIP protocol can be used in a much more broader sense). In it's most "purest" level, unlike with Skype's closed protocol, SIP lets two (or more) connect with each other without having to worry about a centralized server, totally independent of a boss who says you gotta ask him first, awesome!. But as with Skype, where we have to use their server, which is a must!.
What the heck is SIP "Witch"??
Well, it's a centralized server that helps to connect two (or more) individuals at the both ends, easily.
Wait... isn't that like with Skype??
The short answer is No!. With Skype, the laws or protocols are applied within their centralized server, not at your Skype client at home nor at the other end. The skype client GUI is just a front-end. As far as I know, Skype's protocols are implemented within their server which is a closed one. So basically, it's only Skype who can "see" what's going on that server... I think you can guess the rest.
Although as said before, with the help of SIP, we can create a secure peer-to-peer only (peer = end user) communication, but it has its advantages and disadvantageous. Although we can avoid that notorious "server" yet the stability/reliability can be a bit hard to achieve + when we connect more than two at the same time then having a server can actually help to boost the process.
It's like trying to meet 10 individuals by literally going to their places. Or we can gather to a common place and the communication will be much easier. Just like that, the more you have to connect simultaneously, the better it'll be if there was some sort of a common gathering place, in this case is the GNU SIP VOIP server called "Witch".
But unlike with Skype there's a major difference with the "Witch" server. It uses the SIP open protocol and is not a closed source like Skype where only developers can inside it.
What!, so anyone can see what I'm doing!...
No, no. They can't. Just because the protocol is an open one it doesn't mean it's "known". In fact, if you want to be anonymous, then you can easily use an encryption, so no one will see your nerdy stuff :P.
Does Skype support SIP?
Yes. Even though they use their own version of VOIP, yet they support the SIP implementation, meaning that you can connect with your girlfriend at the other end who uses a GNU VOIP client (smart girl :D) with you who happened to be using Skype.
Well, it's interesting what this GNU touch can do to the current domination of proprietary VOIP. Only time will tell :).
You can read more about from this GNU SIP Witch 1.0 official announcement.