Download and install Skype to use your computer to do long distance voice calling: Skype also supports video chat and file and screen sharing. Skype is useful for occasional conference calling. It works best if you have a headset, to eliminate echoes.
(June 2012) Several friends and I tried out Skype for a video chat for a reunion meeting. It worked: we had five people connected together, spanning the USA. (You have to pay extra to have video chat between more than two computers at once.) We had several problems. The user interface is very confusing: getting everybody on the same conversation was complex. We never did see video from some of the participants. Audio was crackly and had strange whooshes. If you want to try this, make sure everybody has the latest version of the Skype software, and practice ahead of time.