I have checked some hours now for a good one.
Best and the simplest thing i have found.
http://www.screenleap.com/
Free, no strings found so far.
No need to waste time installing or registering anything.
Works perfect with "share are only" if you have multimontor setup.
Most other software shares full screen default, and you cannot choose which monitor you share.
You need to be aware that it does not encrypt data. Check FAQ.
But if you have to run voice over Skype/Mymble/Whatever separate anyway..
What harm can a hacker do by simply getting 50% of the conference ?
Pff.. Oh you wished for screen sharing.. this one does not have it.
So try: http://www.mingleview.com/
It has that multimonitor "bug" and you need to install a simple pluginlike thing.
But "take control" is there and it's very simple also.
Most other software has bundled unneccesary functions like calendaring, document system, filesharing which i already have solutions for.
It's like developing games on excel macros..
unneccesary.. developers please learn that already !