It looks as though there is a web-based (flash) version of iCall that should work on the Mac.
I'm not aware of any out-of-the-box solutions for free calling, but with a combination of different products it is possible to get free calls to the US and Canada.
The
Gizmovoice has some instructions on how to do this. If you're not in the US you will need a US-based phone number and proxy to activate. Ignore the Gizmovoice advice about buying one of their numbers, i just applied for a free one from
IPKall (took a day to get it) and then cancelled the account after signing up with Google Voice.
Although the Gizmovoice page says calls are limited to 3 minutes this only applies to calls made by dialling from the Gizmo (or SIP) software. If you log into the Google Voice page and initiate the call from there you can talk for a lot longer.
There are Dashboard
widgets and
standalone applications that supposedly make logging into Google Voice unnecessary, but i haven't had much luck with them so far.
