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I'm not sure about live...however Camstudio is an open source tool for screen capture that is free and I think it is pretty good. There is also Jing for a proprietary fix. For a live session you will likely need to subscribe to a webex site or give Big Blue Button a try.
Live is the difficult aspect. If you want to go live you need a conferencing tool. Academically Wimba, Elluminate and Adobe Connect are frequently used. Blackboard has a new Collaborate tool.
I personally use Jing for short demonstrations.
Try http://www.anymeeting.com Its free/low cost and records session. I've only had a play around not as good as Elluminate but that costs quite bit I understand.
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Screenr is petty good for making short films of your desktop with voice added.
I've been satisfied using http://mikogo.com/ for groups of 10 or less. To stream it to the masses you can use something like http://livestream.com/ but quality is not going to be very sharp, especially if you're screencasting your entire monitor.
What is with WINK ( Windows) - it is a gift ( free) from Satish Kumar: http://www.debugmode.com/ I like it..: