Jeff Pulver, voice-over-IP impressario, has announced free computer-to-telephone calling to numbers in the US and Canada (and, it appears, also to Israel and the U.K.) for the holidays. To take advantage of this, you need to sign up for sign up for FreeWorld Dialup (free) and install an IP telephony application like X-Lite (also free).
I’ve gotten this to the point where I can dial out, but using the Mac version of X-Lite, my voice sounds “robotic” to the person I’m calling (however their voice sounds fine to me). Brother Johnny called me from the Windows version of X-Lite and things worked well (although because he was using a simple microphone and PC speakers, there was an annoying echo).
If you want to dip your toes in the water of VOIP, this is a good way to start.