skype rocks (for the most part).
+1 for Skype. it is the easiest way to do video chat between Macs and PCs, plus it is very easy to do file transfers. Mac to phone calls work well most of the time, better for calling international numbers, for some reason. Calling at US numbers does not always work reliably as the calls are frequently marred by interruptions, static etc.
Unfortunately, the use of Skype is banned at many corporations, precisely because it is so secure that their IT departments can not tap into your conversations.
iChat, on the other hand, is c%ap. If you say it is not, try to establish a video chat, using your iChat account, with your overseas PC user relative who is not computer-savvy.
EDIT: for those who are worried about skype credit: I have been using their paid services for about three years by now and so far have not had any problems with payments etc. I even have a Pro subscription, which gives me an actual US phone number as well as voicemail. I haven't been using it as much while in the US, but a US phone number has been a life saver for me while traveling overseas. Disclaimer: it is just my personal experience - YMMV.