Voice over IP (VoIP) has been largely a Windows/PC thing, but now that appears to be a thing of the past. Skype has released the final version of its software for Macintosh and Linux, bringing VoIP to these other platforms. As a royal Linux user for many years, all I can say is yeah!
Skype had previously released both versions in beta form. The final versions are fully functional, including a full address book, the ability to manage your presence from different locations and the ability to send files. Linux and Mac users also have access to SkypeOut, which allows Skype users to make calls to regular phones, instead of being constrained to computer to computer calling.
To run Skype for Linux, you will need to be running a fairly modern distribution. Certainly, its been tested on SuSE 9, Gentoo 1.4, Debian "unstable", Fedora Core 2, Sun Java Desktop System Release 2 and Xandros. There are certain requirements as regards correct glibc and Qt libraries, so be aware of that before you start.
Similarly, there are requirements for the Mac version. Skype for Mac OS X 1.0 requires a Macintosh computer with Mac OS X v10.3 or newer, a G3, G4, or G5 processor, a minimum 400 MHz processor, 128 MB RAM, 20 MB free hard drive disk space, as well as the usual multimedia bits and pieces.