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yes, try camtwist:

Here's a great video tutorial made by CamTwist explaining the basics to using it: http://www.youtube.com/watch?v=0uMUTvI3NAo

CamTwist is fantastic for screencasting on a mac. Works great on http://ustream.tv

Which is the better service for live demos? Are they both free for commercial purposes?

You should try both and see what you like best. Mazy works for Ustream, I work for Justin.tv.

How about Linux?