Audio
Just tried iChat AV with Kam (audio only, 'cause neither one of us has a video camera); not great signal, perhaps 'cause she's on dialup at the moment rather than broadband, but pretty cool. Free phone calls!
I think it would work better, though, with a headset (earphone and microphone) and broadband on both ends.
Now I want to try out videophone. Someone at work had an iSight (teeny new Apple video camera, perhaps a little bigger than a D-cell battery); not quite willing to plunk down $150 for one yet, but maybe at some point.