I've had each of these on my "to-try" pile for a week and haven't gotten around to it, but they each look promising and you might want to give them a go. Both offer web-based cross-platform (Mac, Windows, Linux) compatibility. Both appear to be free.
LiveLook - http://www.livelook.net/
Easy as 1-2-3
(1) Click "Show My Screen" button.
(2) Give the randomly generated number to your Party.
(3) Ask your Party to enter the number into "See A Screen" box.
Show anything, to anybody, anywhere in the world, instantly.
WORKS ON ANY COMPUTER
Works with any browser on any computer.
Launches instantly from a single click.
NO SOFTWARE NEEDED
All you need is a web browser.
You remain in control of your screen at all times.
WebHuddle - https://www.webhuddle.com/
With the high cost of travel and shrinking budgets, most organizations today are already meeting virtually, or are considering it. But not all web conferencing solutions are created equal. We’d like to introduce you to an alternative way to communicate -- one that makes it easy to meet with the people you need to, when you need to -- all it takes is a web-enabled PC and something to say. You can try WebHuddle right now or see a demo.
Designed for ease of use, WebHuddle overcomes many of the common challenges faced by other virtual meeting applications on the market -- such as high-price to entry, compromised network security, large client downloads, and unreliability -- resulting in a frustrated IT department and less than satisfactory business results.