sip
I must say I don't understand why skype causes so much excitement. I do have a skype account (which I haven't used for a couple of years), and as far as I'm concerned it's yet another proprietary protocol to encumber ourselves with. Do't we have enough of these already ... ?
VOIP is useful; at the moment I only use sip based services (which have obvious security implications) but I can live with this for the time being (I have the security of pstn / mobile to fall back on when needs call for...).