Skype is also nice for going cross-platform, as it works on MS, Mac, Linux, & most mobile devices.
I found it most useful when I was travelling Europe. Because it allowed me to have a local (to where I live) phone number that my friends & fam could call me on, a voicemail box, and the ability to call regular phones. For much cheaper then a calling card or Int'l plan on my cell would be. It's also great for calling fiends & fam that are overseas, regardless of they have skype.
peter