Sure. You can use Apple's program, iChat, though I'm pretty sure this only works with other iChat users (someone correct me if I'm wrong there), or you can use other programs like Skype with no problem. No .mac required at all.
... I'm not very familiar with iChat but what i've seen is that you connect to iChat only and only if you have a .mac account (i'm not sure tho); but you can use your built-in isight with Skype, aMSN and other programs accepting the iSight; read the specs on Yahoo Messenger and AIM to know if they accept iSight webconferencing
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