Talk is cheap, or so they say. At least it is with Google Talk beta ... Google's new IM /voice client! You can check it out at www.google.com/talk/ - I have yet to try it but I'm going to go ahead and download it right now. It seems like this might be one of the new features of the upcoming Google Desktop.
While officially it is only Windows supported right now, it seems that it works with third party IM clones as well. For example, Adium (blech), Gaim (shoutout to Sean), and iChat currently all support a couple of its features - though only the official client supports them all (more specifically, the voice features).