Looks like the free VoIP application Skype (from the kazza people - I mentioned it last September) will be coming to OS X. From the Skype forums:

May 18th: In response to all the requests: there WILL be a Skype version for Mac OS X. No info is publicly available about when exactly it will be out, but it's already in the work

May 24th: I will contact you when we have something to send to you, which is not in next week but quite close. Thanks for patient waiting. :-)