Skype-like systems are definitely important nowadays. The problem is that is is generally very well spread on Windows, but much less on other OS, like Solaris for example. If you are interested in, I encourage you to check this out:
https://imo.im
I've tried it and it worked. I can not say it worked as nicely as Skype on windows, but still, it was somewho not too bad.
For those who are more will to get their hands dirty, there is always Sailfin, a communication oriented version of GlassFish ! :)
Set your own server, link it with your own tools, I did not try it myself but I can say I've seen nice demos !
