James Fallows has a good introductory article about Skype, a VoIP service that has 10 million users in 212 countries. As Cory mentioned earlier, an OS X beta is available.
You can also reach people who don't use Skype, through a new service
called SkypeOut. This allows you to dial nearly any cellular or
land-line telephone number in any country and talk. Though it isn't
free, it's really cheap. Skype's prices are in euros – its founders are
Scandinavian, the main programmers are Estonian and its headquarters
are in Luxembourg – and they average two or three American cents a
minute, at any time of day. With a credit card, you buy calling time in
units of 10 euros ($12.18), which are deducted automatically as you
talk.I started with 10 euros. After my wife talked to her sister in Italy
for a half-hour and I made one quick call to the Philippines and five
more within the United States, we still had 9.10 euros left.