Skype is a hugely popular VOIP service. In fact according to some reports, they now have passed out AT&T as the largest carried of international voice calls.
The main reason that Skype is popular is because it is really easy to use. However, until recently the Skype platform was very closed. There is a Skype API, but this only allows you to manipulate the Skype client running on your machine and does not allow you to do anything really interesting like connect Skype to/from another network.
This might be about to change because Skype have recently announced a Beta program of their SIP support. This would open a lot of interesting possibilities ...
Right now it is a closed Beta with limits on who can participate, so I will have to wait some time to try it out.