iChat should work well, IF everyone on the conference call is using a Mac.
When I talk to my parents in Scotland, I use a combination of Mercury for MSN video (webcam) support and Skype for audio.
It's clunky but it works and they don't have to install or learn another application, although it'd be handy if MSN or Skype would support both video/audio.
Skype is supposedly working on a video/audio client for the Mac.
If you need a more 'serious' solution, then a Google search should turn up a number of different Mac/PC options - most will require some $.